Add option to consolidate rules with identical selectors in the CSS output.

[wisniewskit]

Right now it's possible that multiple rules from multiple stylesheets end up in the final output, despite all sharing the same selector. For instance, there could be three separate body { /* etc*/ } rules, even though one would do in the reduced test-case.

Since consolidating in this way can end up re-ordering away the problem being reduced, this should be an option, not a default.