Color improvements

[thecodewarrior]

This PR makes a few improvements to Aha's color handling:

• It creates a test program test_print.c which outputs a page of formatting codes to test Aha with
  
• It replaces the old highlighted flag with proper 4-bit color support, including .bg-highlighted for independent foreground/background highlighting. Other than that I made sure to maintain CSS backward compatibility, which is why I stuck with the old .highlighted.<color> classes instead of doing separate classes for the normal and bright versions of colors.
• Fixes a bug where a non-4-bit foreground color and a 4-bit background color would end up placing the 4-bit background class (e.g. bg-red) in the tag's style attribute instead of the class attribute.
• It supplements the built-in color schemes with configurable ones using the --colors parameter

Because highlighting no longer uses a CSS filter and instead is a separate color, at the moment the built-in color schemes have no normal/bright distinction. I didn't want to go ahead and decide on a color scheme since, in the end, it's your project.