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

Remove code which supports R15

Add custom log rotator option and hourly log rotation

Take a stab at OTP 18 compatability with macro logging

Handle yet another stacktrace form

Sometimes the stacktrace is a larger array of {M, F, A, Prop} tuples.
The handler code was expecting at most 2 tuples in the list. Relax this
requirement and add a test that provokes this edge-case.

support logging with macros per level

Dont flush notify msgs at overload

add reverse_pretty_stacktrace env var, default true

Handle OTP 20 gen_server crash when the client pid is dead