Support scheme-relative URLs

Any chance support for [scheme-relative URLs](http://paulirish.com/2010/the-protocol-relative-url/), e.g. `//example.com/` can be added? Currently these links break in the source view.

To fix this, you could simply replace `//` at the start of any URLs with `http://` if the original page was served over HTTP, or `https://` if it used SSL.
