* add the HSL values of each of the 256 terminal colors
* add color_pair_cache_hash_str()
* move common code to _color_pair_cache_get() helper func
after hashing a string to a color, return the closest matching
terminal color using euclidian distance of the HSL diff vector (this
method was found empirically and seems to work well enough...)
Just to be on the safe side.
Probably only relevant for unit tests where ncurses vars are not
initialized with real values.
Fix unit tests on all platforms.
