Column width

- [new] [`column-width`](https://developer.mozilla.org/en-US/docs/Web/CSS/column-width) property mixin, to suggest an optimal column width to the browser
- [fix] [`rem`](https://github.com/guardian/guss-rem) function is no longer a dependency
- [misc] mixins are ordered alphabetically

Also added links to MDN documentation for each mixin.

/cc @phamann 

![ ](http://media.giphy.com/media/XwTPSEOq66dTa/giphy.gif)