Add RGB -> indexed downconverter

[zyga]

When using RGB colors on a terminal that doesn't support this feature (e.g. screen) the output is pretty hard to read (can be close to useless depending on foreground and background color choices).

It would be much better if we had a way to lookup closest indexed color for any RGB color. This could be glued to terminal capabilities that are explored in other issues.

[zyga]

This is largely implemented now. I need to re-visit a few more things but I like how it works already. I've tested that existing 24bit RGB based application can be correctly operated with an indexed color filter.

The only thing that I wish I did now is to add support for named palettes so that application can install a color palette and provide both RGB and indexed color values that are optimized for color perception and not computed at runtime. This would allow for the automatic down-conversion to only apply to parts of the application that are not optimized for indexed environments. I'll file a few more issues to explore the idea of color names and to add a mechanism where application can install additional named colors easily.