Make yt plugin accessible via /jsonrpc.js interface

[ElFishi]

May I revisit an idea floated already in issue #33 to add API support to the LMS-YouTube plugin?

I would find it quite awesome if I could launch a YT video or playlist by sending the respective ID to LMS' jsonrpc.js interface. No need to replicate the entire functionality from the GUI. Just sending a playlist to a player for it to play.

Thx for your consideration!