Notifier that executes a custom command when an episode is added to library

Cleaned up pull request https://github.com/midgetspy/Sick-Beard/pull/599.

Similar to extra_scripts, I developed this plugin while unaware of that configuration option. The problem with extra_scripts is mostly that it is invisible to the user unless he knows what he is looking for.

The best solution might not be this pull request per se, but maybe combining extra_scripts with this frontend to provide visibility to the user looking for such an option.