Move styles to SCSS instead of Stylus

[klasjersevi]

I'm developing a theme and also making a bootstrap version of quill. Even though Stylus is cool, I would very much appreciate if quill styles was based on SCSS instead of Stylus. It would be much easier to maintain third party development since SCSS is a more popular CSS precompiler and therefore is easier to integrate with other frameworks.

I'd be happy to submit a PR if you want.

[abierbaum]

This would make it much easier to make custom styles.

[Xinatorus]

I've oppened PR #4022