feat: State class loop mixin

[zastrow]

This addresses issue #68 by adding a new feature, the sparkle state loop mixin. This mixin is a private mixin for the color system to generate state-based class names and work with the media query class suffixes. A test has been added to verify the mixin output.

To verify this code:

1. Pull down this branch
2. If it has been a while, run npm install
3. Run npm start to generate the CSS file
4. Run npm test to verify tests pass
5. Open the W3C CSS Validator
6. Upload the generated CSS file: ./sparkle/styleguide/docs.css
7. Verify that the CSS is valid

[mdt2]