Allow unknown non-{}-block at-rules

[seven-phases-max]

Fixes #2770.

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This does not fix #2639 (i.e. non-block directives do not bubble).
(I suppose enabling bubbling for all unknown at-rules by default would be fine, so I was thinking of doing this in this PR too. But since I don't quite understand the bubbling implementation itself and can't perform any deeper debugging I guess I'll leave this modification for someone else (after all it's a bit beyond #2770 scope anyway and more a subject of #2639)).

[matthew-dean]

I don't think unknowns should necessarily bubble. They're unknown. Someone may want them in the output at that location.

[seven-phases-max]

Someone may want them in the output at that location.

Mmm... Yes, I guess no decision is possible to meet them all.
Technically the only two "standard" directives (@charset and @namespace) are subjects for bubbling (at least to not produce not-quite-valid-CSS when a plain css file is imported via nesting like in #2639). But all those whatever other non-block at-rules come from the pseudo-CSS languages (Polymer, PostCSS plugin dialects etc.) and there's no widely adopted pattern (E.g. @apply is a nested directive and should not be bubbled. Contrary most of such at-rules in PostCSS dialects are global only, thus they will expect them to bubble when they appear in wrapped imports).

[lukeapage]

Nice change btw ;)