Add important parameter to the edit() mixin

Hey all,

This addresses #351.

With this you can pass an optional parameter through to `edit()` and it will output the usual code but with an `!important` flag. Useful for when your design has a lot of backgrounds set on elements that make the grid hard to see even with `edit()` active.

The tests pass, and I've checked both the Scss and Stylus versions on CodePen and they both work as intended.

Let me know what you think :smile: