Account for attribute selectors in open brace lookahead

Recognise selectors of the forms [attr=val], [attr*=val],
attr~=val] and [attr^=val] (optionally with single-quoted values).

These selectors weren't previously recognised, so applying @noflip
would just result the first property being ignored (instead of a
rule-wide noflip). A dditional properties still got flipped.

The (simplified) input that triggered this bug was:

    /* @noflip */ figure[typeof*='mw:Image'].mw-halign-left { left: 10px; float: left; }

which became { left: 10px; float: right; } when run through CSSJanus.

Inspired by a patch from Roan Kattouw to the PHP-port of
CSSJanus that ships with MediaWiki:
* https://github.com/wikimedia/mediawiki-core/blob/1.23.3/includes/libs/CSSJanus.php
* https://github.com/wikimedia/mediawiki-core/commit/e5096f2500