Convert 256 color palette to 16 color palette

[thehunmonkgroup]

VIT 2.x uses the urwid display library, which includes support for 256 color and 16 color modes in its palette.

Currently only 256 color mode is supported. Since the color information for tasks is derived from Taskwarrior and needs to be converted, it seems the only sensible way to provide 16 color support is to convert the colors read from Taskwarrior config via some algorithm.

Least mean square seems like a reasonable approach to this issue.

[Matt-Deacalion]

Another option would be to have a configuration setting for the colour modes. Then all the colour modes in urwid could be supported: 16, 88, 256 and 2**24.

Personally I would like this as it would give me more range in making some themes.