Detect and follow CSS @import directives

[mbostock]

If you have

<style type="text/css">
@import url("./test.css");
</style>

we should detect this as a file attachment and have it function correctly, as we already do for <link>. In addition, we should follow transitive CSS imports. Related #415.

[mbostock]

Related #659.

[Fil]

Ideally this would help importing npm:leaflet/dist/leaflet.css (or some other npm file) fromstyle.css (was #1195)