Make colors customizable or at least readable

Currently color values are hardcoded. To make matter worse the used colors
make assumptions about user terminal colors. Resulting text can be unreadable.

Suggestion: Don't use terminal color values at all, that is `color1` to `color15`, but prefer only well behaved color values from 256 color palette (`gray1`, .. etc). Alternatively, make it possible to customize colors by setting options in `.taskrc`.