This adds support for the EXA_COLORS environment variable, and defines a bunch of exa-specific two-letter codes that I pretty much made up arbitrarily that control parts of the interface.
Fixes#160, which I didn’t expect to actually fix this release cycle, but it unexpectedly became easy to do!
LSColors used to be built up from an iterator, and then queried later. But because the resulting HashMap gets queried in serial anyway, we might as well pass in a callback instead, saving the allocation.
This is also technically a little faster because styles that don’t map to anything (like `zz`) are no longer parsed.