Ability to use REM instead of EM

[Mark]

As the title says' I would rather use REM rather than EM to ensure consistency.

[zspecza]

There's no point to adding this because media queries always conform to the root context - R(oot)EM is used for when you need an easy way to access the outer-most context base font size when you're nested deep within child selectors. In the context of rupture, 1em === 1rem