Add carriage.return scope; generate content style

[vmg]

This PR adds support for customizing how we render CR characters on the syntax themes.

cc @jonrohan @aroben

[jonrohan]

Yes, thank you @kivikakk & @vmg 👏 I'm gonna pull this down today and make sure it doesn't break anything else, but so far looks good.

[kivikakk]

One thing I would change is carriage.return to carriage-return. I'm not at a computer now (:bed:), but if you like, please feel free to make that change to the PR!