Add support for transcripts

They're accessed via the following InnerTube endpoint:
https://www.youtube.com/youtubei/v1/get_transcript?key=AIzaSyAO_FJ2SlqU8Q4STEHLGCilw_Y9_11qcW8
And requested via "params": <protobuf encoded video-id>

Notes:

• research is required to know if this could replace the legacy caption endpoint (do their content differ?)
• A videoJS plugin need to be written for those
• They also require quite some work on the UI to integrate well.

Issue opened on the behalf of @syeopite, and thanks to @TiA4f8R for the screenshot :)