Strategy to handle terminals without 24bit support

[sharkdp]

We distinguish 3 modes:

• no color
• 8 bit mode (\x1b[38;5;167m)
• 24 bit mode, aka "true color" (\x1b[38;2;255;0;127m)

Note that we ignore the 16-color ANSI mode, as we can't really predict how the 16 colors look like.

We choose the mode in the following way:

• If --mode=24bit or --mode=8bit => use the given mode.
• Otherwise, if the output goes to a non-interactive terminal => use no color mode
• Otherwise, if PASTEL_MODE=8bit, use 8 bit mode
• Otherwise, if PASTEL_MODE==24bit or COLORTERM==truecolor or COLORTERM==24bit => use 24 bit mode
• Otherwise, use 8 bit mode and show a warning.

Further literature: https://gist.github.com/XVilka/8346728

[sharkdp]

This can now be easily done with ansi::Brush