Colorized mode

[fdcote]

This feature is a generalization of the current dark mode. It enables users to replace the black and white colors of documents with custom colors to achieve a seamless viewing experience. See #38.

Usage notes:

• A keymap (z in the implementation) enables users to toggle between normal (light) mode and colorized (dark) mode.
• The parameter "colorize" sets the colorized mode as the default mode when opening a document.
• The parameters "white" and "black" correspond to the colors used to replace white and black, respectively.

[freref]

This is cool, I'll add some documentation for it and merge it, thx!