Override implicit OR explicit default if `require=True` for `input_option()`.
Merge pull request #95 from mrmachine/require-overrides-default

Override implicit OR explicit default if `require=True` for `input_optio...
Do not write track-level genres if source is not 'track'.
For 'Various Artist' compilations without a usefull 'album' genre, pick the most used track / track-artist genre.
Try to guess 'Various Artists' genre based on track genres if source is 'track'.
Do not write track-level genres if source is not 'track'.
Fixed error message for invalid config option.
Merge pull request #96 from pscn/master

Addition to pull request #92 (lastgenre) regarding track level genres and Various Artists compilations
docs & style for #92

We already have a nice plurality() function in beets.util to calculate the most
popular value of something, so I reused that.