Support for M3U playlist files

[SteveTalbot]

When I browse the music library in Volumio, M3U files show up in the browser, but clicking on them does nothing.

When I click an M3U file, I expect all the tracks in that playlist to be added to the play queue, in the same order as in the playlist.

I have created a modification to the MPD music service plugin, which adds support for any playlist file format that MPD supports. Tested and working with M3U files on Volumio for Raspberry Pi. Pull request to follow.