mpris: use better separators in command lists
mpris: give full syntax in command list
mpris: categorize subcommands
mpris: indent command lists for clarity
mpris: usage: remove notify-* subcommands, they're listed anyway
mpris: comment the format regex
mpris: add --help, describe printf formats
mpris: also match 'mpris help'
mpris: move subcommand lists to help()
mpris: missing subcommand is an error
mpris: adjust usage