Added min_weight option to lastgenre plugin, to provide for filtering out unpopular genres.

I like the 'multiple' option for lastgenre, but the tags pulled from lastfm didn't always make sense.  I added a 'min_weight' option, so that unpopular tags can be filtered out by weight (range 0 to 100).  A setting of 50 gave good results.  Default is disabled.

Cheers,
  Peter