Add binding for listfiles command

[yktoo]

There's currently no function for the listfiles mpd command.

listfiles {URI}

Lists the contents of the directory URI, including files are not recognized by MPD. URI can be a path relative to the music directory or an URI understood by one of the storage plugins. The response contains at least one line for each directory entry with the prefix “file: ” or “directory: “, and may be followed by file attributes such as “Last-Modified” and “size”.

The only way to list files from MPD's database is to use the GetFiles() function that retrieves the entire file list at once. When the list is large this may introduce a performance penalty.