- Made clear fix mandatory for floated grids.

- Added ability to make base-lineheight smaller than font-size,
allowing type to line up on a more granular baseline.

- Added Link color variable.

- Fixed darker-color variables.

- Made button mixin a little easier to manage.

- Moved grid variables into _variables.scss

- Improved body font-size/lineheight calculation.

- Moved island/media block to extend.scss

- Made type-space-sc mixin more efficient by removing unnecessary third
and fourth values.