feature request: property aware normalization

[chriseppstein]

It would be really useful to have a way to normalize values for properties. This would include doing unit conversions, string case conversion, and whitespace normalization in such a way that two values that are always equivalent in the browser are the same string value.

This would probably precipitate a property aware value ast and stringifier.

[soluml]

As noted in the original opticss issue, postcss-colormin, or really just colormin, already handles this for the color property. While normalizing all values would be great, color is a particularly useful property to normalize with it's many different ways of defining the same thing.