Improved color map scalability.

Obsoleted unscalable color maps ${ZESHY_COLOR}, ${ZESHY_PROMPT_COLOR},
and cohorts. Support arbitrary color name permutations for
user-configurable color maps ${ZESHY_STYLE_TO_COLOR_NAMES} and cohorts.
While the codebase naturally remains thunderously broken, the leaky ship
should be patched up with duct tape and sealing foam in a wee jiffy.