Fixes#288, but more-or-less as a side-effect.
The “mi” key in LS_COLORS was meant to be used for a missing link path, but it wasn’t really used like that. There was also a bug where control characters in a broken symlink’s path were assumed to be underlined, because that’s what happened in the default colour scheme, but this assumption doesn’t hold when colours were disabled.
The solution to these was not to introduce another configurable colour code, but to start using _overlays_ to alter a bunch of colours at once. The “mi” code will have to be added back later.
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!
