Add filename tag to mpd module

This small tweak adds a {filename} tag to the mpd module that returns the basename (sans file-extension) of the currently-playing file. The string is filled *only* if {title} is empty, that way a format string like "[{artist} - {title}]{filename}" will return {filename} only if the other two tags are blank. Useful in cases where the currently-loaded file lacks ID3 tags/metadata but the user still wishes to be able to identify the current track.