Add tokens to output

[irudoy]

This PR adds the ability to get raw tokens of cleaned CSS.
For example, you can extract all used selectors from it.

[leeoniya]

in general, i was considering returning some form of the tokens (#7) but not in the raw mutable, unfiltered internal format which is optimized for selector testing but not great for consumption. i don't think a flat structure is ideal either.

feel free to open an issue please use #7 to discuss the output format and what should be the option that triggers its generation (i don't want to take the perf hit of generating it unconditionally).

[leeoniya]

@irudoy

you might be interested to know that v1.0.7 now returns {css, sels} where sels is a Set of all retained selectors. also somewhat related is whitelist accumulation: https://github.com/leeoniya/dropcss#accumulating-a-whitelist