Add no-tags to, and clean up, tmux/music.sh.

tmux/music.sh
	-Add support for tracks without `id3` tags, by simply inserting
	the basename (sans extension) of their file.
	-Clean up code by adding `parse_cmus()`, which cut down on the
	frequently repeated `$(echo "$cmus_info" | grep -oP "...")`.
	-Add some comments to `cmus_info`. (A bash script really needs
	them...)