Adds support for color picker in css`` blocks

[au5ton]

This adds support for CSS colors in the following formats:

• rgb(...)
• rgba(...)
• hsl(...)
• hsla(...)

I also added some rudimentary tests for the color provider as a result of these changes.

Fixes #319

[au5ton]

@runem What do you think? Is there anything I can do to improve this before it can be accepted?