mpdthumb: Add support for embedded cover art.

mpdthumb: Add support for embedded cover art.

MPD has recently gained the "readpicture" command in order to retrieve
cover images embedded in files.  This works very similarly to albumart.

We now have the necessary features in mpdexec to implement this in
mpdthumb.  When the --embedded option is supplied, the script will
first try readpicture, then albumart for each file specified.