Add colors to interactive git-clean

Show header, help, error messages, and prompt in colors for interactive
git-clean. Re-use config variables for other git commands, such as
git-add--interactive and git-stash:

 * color.interactive: When set to always, always use colors for
   interactive prompts and displays. When false (or never),
   never. When set to true or auto, use colors only when the
   output is to the terminal.

 * color.interactive.<slot>: Use customized color for interactive
   git-clean output (like git add --interactive). <slot> may be
   prompt, header, help or error.

Signed-off-by: Jiang Xin <worldhello.net@gmail.com>
Comments-by: Matthieu Moy <Matthieu.Moy@imag.fr>