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
```
If anybody is offended by such an HTML hate crime, we could probably fix it in a different PR and add some more rules to /grunttasks/replace.js where we search the _server/templates/pages/dist/**/*.html_ files for `id="-"` and replace with "" (empty string).

But that is the context behind those errors/warnings and why I disabled them. And in typing this out, I now see how lame my "solution" was since it would effectively mask actual duplicate IDs everywhere.

Or maybe we solve it by making another htmllint task target and set specific ignore rules for the localized markdown in _server/templates/pages/dist/**_.