* add support for colorized ls on openbsd
* add changelog line for colorls support
* add readme line for colorls support
* determine ls command at runtime, don't cache it
* eliminate __fish_ls_command function
This was removed from fish-site in
7c19bf2cc9a3742346527cd6979671f16b8caeb9 because it's out of date, so
it gives a bad first impression.
In my tests it also loads very slowly and features oh-my-fish instead
of a stock fish.
We use sphinx with rst for our documentation, and github supports rst
here, so it seems weird to have markdown just for these.
It also allows us e.g. to include the CHANGELOG in the docs without
requiring another build dependency.
