auto_link doesn’t support protocol-relative URLs

[barnabywalters]

Auto-linking this input:

'//example.com/photo.png'

Produces this output:

'//<a class="auto-link" href="http://example.com/photo.png">example.com/photo.png</a>'

It also re-links photo URLs inside <img> elements:

'<img src="//example.com/photo.png" alt="" />'

becomes:

'<img src="//<a class="auto-link" href="http://example.com/photo.png">example.com/photo.png</a>" alt="" />'