Improve link identification.

Implements an idea from poole/poole#62, where color blind UAs wouldn't
be able to easily identify URLs in the body text of a post, without
'scrubbing' over them manually.

Basically this triggers the underline on two conditions:

  1. When a UA has their cursor over the post block. This doesn't help
  when you're dealing with mobile/touch devices, soooo...
  2. Trigger the event on a media query, too.