Add support for "pluggable" loggers

[defect]

To support more logging destinations than collins, this adds a loggers config key which is used to load code from tasks/modules/loggers. The goal is to have a way of adding logging to various external destinations (elasticsearch for instance) without having to build them all in to the gem.

By default, tasks will log to stdout and syslog. Users can then specify logging classes in tasks/modules/logging to load and the log line will be sent using those as well.

@roymarantz @Primer42 @byxorna

[william-richard]

first glance LGTM

[byxorna]

LGTM

[roymarantz]

:+1: