mpd: provide launchd plist

This adds a launchd plist for mpd.

Using this plist, mpd will search standard directories for it's configuration (most notably `~/.mpdconf`). Ths is mentioned prominently in the manpage, so I don't think we need to put it in the caveats?

I've also included paths for logs (similar to how most formulas seem to handle it), although I have noticed that launchd fails to create the files if `#{var}/log` ist not present.