Add a simple .grid toggle

[badlydrawnrob]

For now you can use <a onclick="toggleGrid()" href="#">Toggle</a>
Currently toggling but need to hard refresh to clear .grid-baseline styles.
It might also be wise to remove the mixin, and use simple css and a class.

When the .grid-baseline() (which I've forgotten how I did) is previewed, it overlays the content so you can't select, click or interact with elements on the page. A simple toggle link would help with this.