Add DNS Prefetching for included external domains

This has been brought twice previously that I can find: https://github.com/h5bp/html5-boilerplate/pull/468 and https://github.com/h5bp/html5-boilerplate/pull/965.

However both introduced prefetching for domains that weren't actually used in the default `index.html` and required customization.

I also don't believe that one of the [original reasons](https://github.com/h5bp/html5-boilerplate/pull/468#issuecomment-1059525) against it applies to this situation.

The reasoning behind this PR is simple: both `ajax.googleapis.com` and `www.google-analytics.com` are used by default in H5BP and would benefit from being prefetched.