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
```
Sample errors with these two rules commented out:
```
// 'Duplicate ID “-”.',
// 'The first occurrence of ID “-” was here.'
```
```sh
server/templates/pages/dist/ta/terms.html [L37:C11] Duplicate ID “-”.
server/templates/pages/dist/ta/terms.html [L31:C11] The first occurrence of ID “-” was here.
server/templates/pages/dist/ta/terms.html [L39:C15] Duplicate ID “-”.
server/templates/pages/dist/ta/terms.html [L31:C11] The first occurrence of ID “-” was here.
```

Where _server/templates/pages/dist/ta/terms.html_ is:
```html
<h4 id="-">சேவை விதிமுறைகள்</h4>
<ol>
<li><h4 id="-">அறிமுகம்</h4>
<p>Mozilla வழங்கிய ஆன்லைன் சேவைகளின் தொகுப்பான, Firefox கிளவுட் சேவைகளின் (&quot;சேவைகள்&quot;) உங்கள் பயன்பாடு இந்தச் சேவை விதிமுறைகளால் (&quot;<u>விதிமுறைகள்</u>&quot;) நிர்வகிக்கப்படுகின்றன.</p>
```