Add grunt-contrib-htmlmin.

Only used for the GitHub docs.

Running "htmlmin:dist" (htmlmin) task
Minified _gh_pages/about/index.html 18.37 kB → 15.38 kB
Minified _gh_pages/browser-bugs/index.html 17.47 kB → 14.48 kB
Minified _gh_pages/components/index.html 298.71 kB → 258.74 kB
Minified _gh_pages/css/index.html 305.7 kB → 277.57 kB
Minified _gh_pages/customize/index.html 142.77 kB → 125.54 kB
Minified _gh_pages/getting-started/index.html 67.7 kB → 62.09 kB
Minified _gh_pages/index.html 11.84 kB → 9.97 kB
Minified _gh_pages/javascript/index.html 199.8 kB → 179.11 kB
Minified _gh_pages/migration/index.html 25.08 kB → 19.61 kB

I skipped the examples on purpose since many people seem to copy paste those. Let me know your thoughts.