Add a :rel filter to links.

This allows us to better describe links for SEO and do things like:

has_link 'Next', { rel: 'next' }
has_link 'Previous', { rel: 'prev' }
has_link 'Sign In', { rel: 'nofollow' }
Fix have_link matcher test descriptions.
Add tests to the have_link matcher with a given 'href' filter.