Use plain Logger

[cjbottaro]

There should be a whole documentation page on how to use Elixir's Logger module with faktory_worker_ex.

With a bit of configuration, it can accomplish some essential goals:

• Set faktory_worker_ex's log level at compile time (using compile_time_purge_matching).
• Set faktory_worker_ex's log level at runtime (using a custom formatter).
• Do this while not affecting other apps log level.
• Add visual queues to differentiate faktory_worker_ex's log messages from other apps.