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)

Perhaps add
```jade
- var helpId = "help-block-" + variable.name
```
And then refer to `helpId`, to avoid the repetition.
(See http://jade-lang.com/reference/code/ for description of `- JS_stmt_here` syntax)