Modify colors a bit to ensure proper contrast with bg color

According to the following a 4.5/1 contrast ratio is recommended.

http://www.w3.org/TR/UNDERSTANDING-WCAG20/visual-audio-contrast-contrast.html
Make the color calculation more configurable
I guess cl is needed now that I'm using reduce and some
docstrings
the color feature is needed for several functions
let terminal emacs users specify the bg color
Merge branch 'ensure-contrast'