The strict configuration work in 31cf6c0 prevents some
backwards-compatible code in lager_file_backend from working as the
configuration is rejected before it gets to the backend. This fix adds
an additional guard on `check_handler_config/2` to support the `tuple`
configuration structure supported by `lager_file_backend`.

* Make lager_handler_watcher test more resilient by scanning for
  messages of interest rather than assuming we know exactly what
  messages will arrive at the test backend

Previously, configuration would come from application:get_env/3 or
proplists:get_value/3. Now we standardize on a single format
for retrieving values from the environment or from a proplist.

This is, perhaps, not the most ideal setup. It still ignores other
sinks but does bring logging back online after the appropriate delay.

Signed-off-by: Brian L. Troutwine <brian.troutwine@adroll.com>

At this point in the work, the killer will correctly stop the
DEFAULT_SINK in the event of overload but our friend never comes back
up and lager stops working past this point.

Oops!

Error message:

=SUPERVISOR REPORT==== 10-Mar-2016::16:18:11 ===
     Supervisor: {local,lager_handler_watcher_sup}
     Context:    child_terminated
     Reason:     killed
     Offender:   [{pid,<0.63.0>},
                  {id,lager_handler_watcher},
                  {mfargs,{lager_handler_watcher,start_link,undefined}},
                  {restart_type,temporary},
                  {shutdown,5000},
                  {child_type,worker}]

=SUPERVISOR REPORT==== 10-Mar-2016::16:18:11 ===
     Supervisor: {local,lager_handler_watcher_sup}
     Context:    child_terminated
     Reason:     killed
     Offender:   [{pid,<0.59.0>},
                  {id,lager_handler_watcher},
                  {mfargs,{lager_handler_watcher,start_link,undefined}},
                  {restart_type,temporary},
                  {shutdown,5000},
                  {child_type,worker}]

=PROGRESS REPORT==== 10-Mar-2016::16:18:11 ===
          supervisor: {local,lager_sup}
             started: [{pid,<0.113.0>},
                       {id,lager},
                       {mfargs,{gen_event,start_link,[{local,lager_event}]}},
                       {restart_type,permanent},
                       {shutdown,5000},
                       {child_type,worker}]

Signed-off-by: Brian L. Troutwine <brian@troutwine.us>

Address #286. This will enable applications to
dynamically configure log sinks, provided they
have already instrumented the code via parse
transformation to use those sinks.

Previously, configuration would come from application:get_env/3 or
proplists:get_value/3. Now we standardize on a single format
for retrieving values from the environment or from a proplist.

This is, perhaps, not the most ideal setup. It still ignores other
sinks but does bring logging back online after the appropriate delay.

Signed-off-by: Brian L. Troutwine <brian.troutwine@adroll.com>

At this point in the work, the killer will correctly stop the
DEFAULT_SINK in the event of overload but our friend never comes back
up and lager stops working past this point.

Oops!

Error message:

=SUPERVISOR REPORT==== 10-Mar-2016::16:18:11 ===
     Supervisor: {local,lager_handler_watcher_sup}
     Context:    child_terminated
     Reason:     killed
     Offender:   [{pid,<0.63.0>},
                  {id,lager_handler_watcher},
                  {mfargs,{lager_handler_watcher,start_link,undefined}},
                  {restart_type,temporary},
                  {shutdown,5000},
                  {child_type,worker}]

=SUPERVISOR REPORT==== 10-Mar-2016::16:18:11 ===
     Supervisor: {local,lager_handler_watcher_sup}
     Context:    child_terminated
     Reason:     killed
     Offender:   [{pid,<0.59.0>},
                  {id,lager_handler_watcher},
                  {mfargs,{lager_handler_watcher,start_link,undefined}},
                  {restart_type,temporary},
                  {shutdown,5000},
                  {child_type,worker}]

=PROGRESS REPORT==== 10-Mar-2016::16:18:11 ===
          supervisor: {local,lager_sup}
             started: [{pid,<0.113.0>},
                       {id,lager},
                       {mfargs,{gen_event,start_link,[{local,lager_event}]}},
                       {restart_type,permanent},
                       {shutdown,5000},
                       {child_type,worker}]

Signed-off-by: Brian L. Troutwine <brian@troutwine.us>

Previously, if you configured a trace to use an
extra sink, you'd get a noproc error. Addresses
issue #296 on github.

Based on code review comments put this function into util.
Added a couple simple unit tests for sanity checking purposes.

During code review we decided that using Sink ++ '_lager_event'
would provide better isolation for gen_event names than
Sink ++ '_event' as we had been using up to this point.

For example a sink called 'audit' is now 'audit_lager_event'
than 'audit_event' which might clash with a user's predefined
own module or process name.

Previously bad configurations would be
silently ignored. Now they will throw
an error tuple.

The default strategy is 'handle', for backwards compatibility,
and is set to that if otherwise undefined.  Other strategies
include 'mirror' and 'ignore'.
   Before when the error_logger was removed when lager starts up,
some functionality was chopped off, such as forwarding io to the
caller's group leader, this is supported with the strategy 'ignore'
and is similar to how the io system in erlang works traditionally.
   Setting the 'mirror' strategy will essentially combine the other
two strategies such that the remote node reply is forwarded and also
handled locally. This is in some cases more in-line with what some
people might expect, efficiency may vary.
   The traditional 'ignore' approach is more suitable and most likely
originally intended for embedded terminals where 'mirror'-ing the io
is largely unnecessary.

Throw {error, bad_config} if we find one during
handler startup.

Finish lager API modifications
Update config API to handle multiple sinks
Identify areas where new code is yet to be written

the default handlers defined at lager.app overwrites handlers defined previously by application:set_env(lager, handlers, ...) if lager application is started application:start(lager)

Adds transparent event stream processing and statistics.
A new 3-tuple trace is introduced as `{Key, Op, Value}`, but
for backwards compatibility `{Key, Val}` implies `=` for `Op`
and `{Key, '*'}` remains as is in the case of wildcards.
A simplified query tree module is generated which reduces
redundant selection conditions to minimize filtering overhead.