Check for newly-revealed nodes based on media query changes

[johnholdun]

The embed feature does not fetch an embedded link if the <a> has the display: none; property, which is handy for avoiding extraneous network calls on mobile devices, for example. If one of these links is revealed after the DOM is loaded as the result of a media query, it should be detected and fetched.

Note that we already listen for orientationchange events, so we just need to handle viewport size queries.