feat(build): Adding htmllint Grunt task

This isn't linked to any `default` or `lint` Grunt task since it adds 15-20s per build, but it's nice to keep around since it may catch some HTML markup issues in the future.

```sh
$ grunt htmllint

Running "htmllint:all" (htmllint) task
>> 697 files lint free.

Done, without errors.


Execution Time (2014-07-24 19:11:53 UTC)
loading tasks   1.7s  ▇▇▇▇▇ 10%
htmllint:all   15.3s  ▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇ 90%
Total 17s
```
Let's add this check to the "en_US" target also, since it should be speedy (grunt-html doesn't mind mustache templates?).