Add grunt-contrib-htmlmin.

Only used for the GitHub docs.

```
Running "htmlmin:dist" (htmlmin) task
Minified _gh_pages/about/index.html 17.61 kB → 14.74 kB
Minified _gh_pages/browser-bugs/index.html 16.73 kB → 13.83 kB
Minified _gh_pages/components/index.html 292.04 kB → 252.42 kB
Minified _gh_pages/css/index.html 290.13 kB → 263.23 kB
Minified _gh_pages/customize/index.html 129.37 kB → 112.76 kB
Minified _gh_pages/getting-started/index.html 64.94 kB → 59.51 kB
Minified _gh_pages/index.html 11.76 kB → 9.9 kB
Minified _gh_pages/javascript/index.html 198.45 kB → 177.67 kB
Minified _gh_pages/migration/index.html 25.01 kB → 19.54 kB
```

I skip the examples on purpose since many people seem to copy paste those. Let me know your thoughts.
Ah, that's a very valid concern. I've run into these validation reports a lot as I write new docs sections.