Add fields

This should close out #3 with the addition of preprocessor and browsers.

@wallawe, Quick thing: in [this commit](https://github.com/travisvalentine/wrap-semantic-ui/commit/0ff45dff56701859e52527363e7e3a430e800252#diff-df73a8de3e8d014198a123a5329eab37R10) you had only shown a theme's first category. I ran into an error with that so I changed it back to use `@theme.categories.each...`. If that's not okay we can change it back.