Add support for .ogv files

I have an old series from the 80s that is only available in .ogv format, but NzbDrone does not recognize it.

I know that this is not a common format at all, but I don’t want to transcode the files since they are already of very low quality.

I should add that ogv is a container format. I’ve tried to put the audio and video in an .mkv container instead, but that creates audio sync issues.

Added to Trello, will try to get that in before the next release: https://trello.com/c/qlQSXdgt/537-add-ogv-extension