Adds ability to configure stylistic alternates across the site

Experimenting with adding configuration to the Sass version. If you want to use Stylistic Alternates right now, you encounter the same issues as you would for any of the other properties: they get overwritten if you use `font-feature-settings` later so they don’t cascade.

This would allow `$opentype-salt` to be turned on, changing over to the alternate characters across the whole site. Not sure what else I would add config for yet, probably just OpenType features that would be useful site-wide that aren’t on by default in the spec.

Feedback is welcome!