Enable box-sizing changes at the component level

Inherit the box-sizing property from the html element, which allows
components to easily override the specified box-sizing such that their
desired behavior works as intended.

This allows Susy to not be concerned with third-party components'
box-sizing, without breaking the desired behaviors.

Working example: http://sassmeister.com/gist/ba636f049722573b3afc

Inspiration: Chris Coyier via
http://css-tricks.com/inheriting-box-sizing-probably-slightly-better-best-practice/
Both parts of this should probably use `@root` so that it doesn't break if someone uses the mixin in nested context.