Autotagger: option to prefer the existence of optional tags

### Feature Request

`match.require` completely ignores any potential matches which lack certain fields. What I'd like instead is a less aggressive setting which merely causes such matches to be downgraded.

For example, suppose I'm importing an album without a record label and there are two potential matches: one with a record label and one without (otherwise identical); I'd like beets to prefer the former. `match.require` will accomplish this -- however, if I import another such album and there is only one potential match (without a record label), beets will ignore it completely.

(Apologies if this has already been suggested. I couldn't find an existing issue.)