Enhancement: Add DNS Prefetch feature

addresses #37 

## DNS Prefetch for External Sources

This enhancement adds a new Soil feature which looks at all enqueued scripts and styles, comparing them to the site url, and adds explicit DNS Prefetch Link to head if it doesn't match. 

https://github.com/h5bp/html5-boilerplate/blob/master/dist/doc/extend.md#dns-prefetching

The only thing I was unable to achieve, according to the linked documentation from h5bp, was inserting this after charset Meta. If anyone knows of a way to achieve this I would be happy to make adjustments. 

I have not thoroughly tested this patch against a full fledged install yet, but it does work on a clean Roots install, only adding a link for the external jQuery. 

It is enabled with `add_theme_support('soil-dns-prefetch-external-assets')` as per the Soil pattern.