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.
Not sure if it would be better to do something like `broad = config['ftintitle']['broad'].get(bool)` before this and then just call `broad` here.