drop unused css variables?

[leeoniya]

because CSS optimization is a slippery slope, currently DropCSS draws a line in the sand for how it can mutate the provided CSS. right now it will only remove CSS selectors, and if all selectors are removed for a specific block, then it will also remove that block. that is to say, it does not touch the contents of the blocks.

dropping unused css variables should be a simple affair with just 2 regex passes, but would deviate from the current block modification abstinence. on the other hand, it still serves the core function of "removing unused css", so maybe i'm making a mountain out of a molehill.