Color support for "git-add -i"

Color support for "git-add -i"

This is mostly lifted from earlier series by Dan Zwell, but updated to
use "git config --get-color" and "git config --get-colorbool" to make it
simpler and more consistent with commands written in C.

A new configuration color.interactive variable is like color.diff and
color.status, and controls if "git-add -i" uses color.

A set of configuration variables, color.interactive.<slot>, are used to
define what color is used for the prompt, header, and help text.

For perl scripts, Git.pm provides $repo->get_color() method, which takes
the slot name and the default color, and returns the terminal escape
sequence to color the output text.  $repo->get_colorbool() method can be
used to check if color is set to be used for a given operation.

Signed-off-by: Junio C Hamano <gitster@pobox.com>