Tie help block to input with aria-describedby

Improves the accessibility of http://getbootstrap.com/customize/ by explicitly tying any (optional) help block to the relevant ``<input>`` using ``aria-describedby``

![capture](https://cloud.githubusercontent.com/assets/895831/5350617/a8a857e4-7f3f-11e4-9811-fb7b77ecebe8.PNG)