VLCTranscoder adaption to be more user friendly

Problem: the options format that we currently have to reencode/ transcode a media file are quite complex for a new developer. In order to make that more accessible we could adapt the Transcoder Api and make it robust enough to catch common mistakes and therefor open it up to a wider audience.

The method that we currently have for reencodeAndMuxSRTFile:(NSString *)srtPath toMP4File:(NSString *)mp4Path outputPath:(NSString *)outPath is too specific

Inspiration can be taken from the Convert & stream interface in VLC for Desktop. Here there are different options for a user to choose from like video codec, audio codec and subtitles and wether the subtitles should be as overlay or channel which are then translated in the options format. Furthermore the user can be prompted based on selection if his combination is even possible. A similar behavior can be communicated by VLCKit. At the same time we deal with a very fragile part of libvlc which shouldn't be added without tests that ensure before we ship a new version that these options still work.