Resolves#25.
Completely refactors the MPD module to be the 'music' module. This now supports both MPD and MPRIS with the same UI for both.
BREAKING CHANGE: The `mpd` module has been renamed to `music`. You will need to update the `type` value in your config and add `player_type` to continue using MPD. You will also need to update your styles.
Adds `on_click_middle`, `on_click_right`, `on_scroll_up`, `on_scroll_down`.
BREAKING CHANGE: `on_click` is now called `on_click_left` for consistency with new options.
Resolves#44.
