Support web fonts (@font-face)

[nicoburns]

Many websites use web fonts. We should support them.

Implementation Plan

• When (just after) a stylesheet is added to the Stylist we should introspect the stylesheet to look for @font-face rules
• We should fetch any font URLs found (for now this can be synchronous as with our other networking)
• If the font is successfully fetched then it should be registered with the Parley FontContext using https://docs.rs/fontique/0.1.0/fontique/struct.Collection.html#method.register_fonts . For now this can not worry about supporting custom names, matching criteria or deregistering.