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.
BREAKING CHANGE: This renames the module from `sys-info` to `sys_info`, and almost every formatting token from `kebab-case` to `snake_case`. Any use of this module will need to be updated.
