Delegate video/audio stream handling to external service

[alexpado]

The global process would be that once the file has been downloaded, the external service would be called with the filename.

1. Receive the filename through HTTP
2. Start processing the file with mkvmerge and ffmpeg (video / audio / subtitles)
3. Copy the tracks into the library and call back the main service with all track data

The main service would then save everything into the database (requires library v2 feature [ #11 ] [ #18 ])

This also requires #31 which would help greatly assigning tasks.

Note: The purpose of this feature is to run the program on any computer for it to be able to convert any media while benefiting from GPU accelerated conversion, which could speed up the process of handling the whole library (currently sitting at 5TB+...). It would also allow multiple instance working in parallel to speed up even more.