Adding hidden htmllint task

Adding a Grunt htmllint task to help find possible HTML errors in our templates.

```sh
$ grunt htmllint
Running "htmllint:all" (htmllint) task
>> 10 files lint free.

Done, without errors.


Execution Time (2014-07-24 21:31:52 UTC)
loading tasks  1.4s  ▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇ 42%
htmllint:all     2s  ▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇ 58%
Total 3.5s
```