Subliminal plugin added.

Subliminal itself check for existing subtitles (even inside mkv videos), so there's no need for a 'overwrite' control in the plugin schema as for Periscope. Language codes instead must be 3-letter ISO-639-3 code. The accept/reject criteria is the same. Subs will have the language code in extension (i.e. "title.it.srt", "title.en.srt" ecc).