Prevent silent dropping of messages when hwm is exceeded

is_loggble - support for severity as atom

Allow the console backend to redirect output to a remote node

Cosmetic Typo

To support tracing to a console that is not the erlang node's *direct*
console introduce 2 additional options to the console backend:
`group_leader` and `id`. Group leader allows to use io:put_chars to send
the console output to a remote pid (eg. the group leader on a node
created by nodetool) and the ID allows multiple console backends to
exist simultaneously (for example {lager_console_backend, NodeName}).

Additionally, to support ephemeral console backends with associated
traces, if the console handler explicitly removes itself from the event
handler, remove any traces for that id.

To facilitate this, a new function has been added to the lager module:
`lager:clear_trace_by_destination/1`.

Fix markup - inline code

Add a wrapper to allow lager to install a sys trace function

Fixed platform_define pattern matching

Fix reinstalling all handlers after killing a sink #458

test erlang/otp 21.0

OTP 21 support

To support OTP 21 we manually start error_logger if it is not present
(and lager is configured to install an error_logger handler) and we add
error_logger as a logger handler.

Longer term we should switch to installing our own logger handler, but
that is a larger task as we'd have to redo all the event parsing, if
that is even possible.

Bugfix. flush_threshold not working.

Fix incorrect option name validation.

Add `node` as a formatting option

Closes #309. This will enable the use of `node`
in log formatting. It uses the value of the node()
function via atom_to_list/1 to output a value.

There is a simple test case.

Update Elixir section with information about parse_transform

According to the official changelog for elixir 1.5 parse_transform is
deprecated since this version. See
https://github.com/elixir-lang/elixir/blob/v1.5/CHANGELOG.md#4-deprecations

use "[{level,info}]" instead

Pretty print lists of records at top level and field values

Ignore return value of lager:dispatch_log in lager.hrl

This is to suppress dialyzer's `unmatched_return` warning

Don't log when we dropped 0 messages

add flush options to proplist validation

Make a few corrections to the recent mailbox flushing changes