Allow adding m3u as file with 'a' command

[phunanon]

(By default, the 'a' command is: Add a file/directory to the playlist)
Currently, the program returns an error to the user stating "You can only add a file using this command." while trying to add a .m3u file to an empty playlist, and a "Please clear the playlist [...]" error for adding to a non-empty playlist.

I suggest: allow 'a' to add the m3u file's list to an empty playlist;
and further: allow 'a' to append the m3u file's list to a current playlist (with the further suggestion of ignoring m3u when found recursively in a sub-directory).

Thanks.