Simple colored bash prompt

The prompt color theme can be customized simply by setting the PROMPT_COLOR envvar.

For example PROMPT_COLOR='2;7' is dim inverse-video and PROMPT_COLOR='1;33;53' is bright/bold yellow with overline.

Set prompt_color_force to force usage/updating of color PS1: by default it is only setup cautiously if $TERM ends in "color" and PS1 is the fedora default.