Update hide-text extend

Another CSS image replacement technique, used by [Bootstrap](https://github.com/twbs/bootstrap/blob/master/dist/css/bootstrap.css#L6112-L6118). I find this a better solution than the previous text-indent solution...
Merge pull request #52 from jessedobbelaere/update-hide-text

Update hide-text extend
Merge branch 'master' of https://github.com/getchopstick/chopstick-boilerplate