Full support for DFPlayer Mini/DFR0299 (specifically the "loop" command)

[Nawor3565]

Hello! I'm working on a project that uses a DFPlayer Mini that needs to loop an .mp3 indefinitely, but the "MP3 Player" integration just doesn't appear to have the command to enable repeat mode (the last one in the "Serial Control Cmd" list). It should be pretty simple to add, and I would put in a pull request myself if I was better with Git.

Thanks!