Support Media Players

[Ramblurr]

Is your feature request related to a problem? Please describe.
I'm always frustrated when I tell mycroft to turn off/on some media player and it doesn't understand.

Describe the solution you'd like
HA has extensive support for media players, I'd like to see this skill leverage that!

Describe alternatives you've considered
none

Additional context

There are several logical stages to take in supporting media players.

First stage

Simple playback commands that don't require an object beyond the entity itself

• media_player.media_pause - "Pause the living room speaker"
• media_player.media_stop - "Stop playing in the living room"
• media_player.media_next_track / previous_track - "Next track", "Previous song" (open question: how to choose which media_player entity when none is provided?)

Volume Controls:

• media_player.volume_up / down - "Turn up the volume"
• media_player.volume_set - "Set the volume to 50%"
• media_player.volume_mute - "Mute the TV"
• media_player.repeat_set - "Set repeat to single track"
• media_player.shuffle_set - "Turn on shuffle"

Future stage

Eventually it would be nice to play specific playlists or albums. Unfortunately this is sort of media player platform specific in HA, that's why I'd leave it for a future stage.

For commonly played media (radio stations, common playlists etc) one could setup a script in HA, then use mycroft to trigger the script.