Add option to restrict the desired encoding when searching for episodes.

I'm creating a pull to notify everyone that this is in progress. I welcome people to test this out (backup your database first) and also to suggest any improvements that can be made to this particular feature.

Due to the size of the patch I will limit the scope to restricting searches __only__. If you have other encodings that need to be supported along with file extensions then please notify me.

I have not tested mass updates nor saving the encoding when episodes are downloaded or postprocessed. That needs a bit more testing than I have time for right now. Please assist if you can.