Add support for unicode-range wildcard minification

[strarsis]

@bramstein; @zachleat: Some unicode-ranges can be even further minified into a wildcard in CSS output (and possibly also in the Unicode Range Interchange tool):

E.g. this unicode-range unicode-range: U+40-4F;

Can be further minified to: unicode-range: U+4?; (As ? stands in for 0-F.)

Originally created in https://github.com/zachleat/glyphhanger/issues/20, but this is the upstream repository.