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
```
Haha. Research indicates that's just sending `stdout` to `/dev/null` (it's so webscale). Silly test to prove the concept...

```
$ echo 'oh hai' &> /dev/null
$ >&2 echo 'all the errors' &> /dev/null
all the errors
```

`>&2` in the second example is sending output to `stderr` which is not being sent to `/dev/null`. Carry on.