Generic event driven hooks to invoke external scripts in unified manner.

[mmhere]

Similar to #7608 it would be nice to have a generic "call this external program" when Notifications occur.

Perhaps this could even be generalized into a "event hook" subsystem that can invoke external scripts when various important things take place in the logic flow inside Medusa.

Hope this isn't overly general. It is.

What I'm thinking is that it would be nice to be able to run a script when things like the following occur, and do it in a generalized manner.

1. Snatch of an .nzb happens and is deposited in BlackHole
2. Subtitle download completed
3. Postprocessing completed

[1] Right now I pile on to libnotify.py with a hack in there that calls an external script upon snatch; this notifies me via Notification mechanisms not yet supported in Medusa. It also massages the handling of the snatched .nzb file before handing it off to the retrieval program (sabnzbd in this case).

[2] Right now you can set subtitles_extra_scripts to do certain handling (I have a script that edits ads out of the subtitle content, and also symlinks them into an alternate location in the filesystem, omitting details/motivations here). The use of subtitles_extra_scripts is, however, handled in a separate location in the Medusa web GUI.

[3] Right now you can set extra_scripts on the /config/postProcessing/, but like the subtitles extra script, this is in a separate place in the GUI. I happen to use this to run a script when download/postproc are done. Here I could also hook into my hack in libnotify.py (for "Notify on download) but...

You see where I'm going with this? 'twould be nice to have a way to hook or trap various events and call out to custom behavior written by local Gurus™ (like me ;-).