Support Regular Expressions and Arrays in cssImportIgnore

CSS file URLs may legitimately include commas. The recommended CSS for Google Web Fonts, for example, might look like:

    @import url('//fonts.googleapis.com/css?family=Open+Sans:300italic,400italic,600italic,700italic,400,300,600,700&subset=latin,cyrillic-ext,latin-ext,greek-ext');

This PR allows `cssImportIgnore` to support regular expressions and an array of strings or regular expression in addition to the current format of a single "comma separated list."

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