Add a support for a .button class

[goodpixels]

Sometimes you want to style your links as if they were buttons - and it would be handy to allow users to assign a class which would inherit button styling, so you can write this:

<a href="http://google.com" class="button">Click</a>

This should render the same as this:

<button>Click</button>

Currently you have to @extend, but I am not sure if it's the best idea, because it affects the compile time.