Option to scrobble only first artist

This adds an option to only scrobble the first artist in case of multiple artists. E.g. Spotify tags tracks with "Artist A, Artist B" if multiple artists collaborate on the same track. This is send to Last.fm as artist="Artist A, Artist B" (a concatenated string). Last.fm does not recognise that as two separate artists, but sees it as one unknown artist. With this option, it will only send the first artist, "Artist A" in this case, keeping the scrobbles to Last.fm clean and correct.
The default option is the current option to concatenate artists. A user must manually change this settings.
Might be very specific to the Spotify plugin. It is set in getMetadataFor() of Spotify's ProtocolHandler.pm So might not be the best to include this patch as such. I wrote this patch a while ago for this specific problem with scrobbling Spotify tracks. But I do not see a way to change this in the Spotify plugin, whilst still keeping the multiple artists for display purposes on the player. So this was by far the easiest hack/fix.