ftintitle: Add option: `broad`.

Determines how lenient ftintitle should be when catching "feat." strings. If set to False, it will not catch "with", "&", "vs", etc. The exact strings are defined in beets/plugins.py's `feat_tokens()`.

Defaults to current behaviour if not set.

See also #1184.
I'm not married to the option name, it was just the best I could come up with offhand.