grep: color patterns in output

grep: color patterns in output

Coloring matches makes them easier to spot in the output.

Add two options and two parameters: color.grep (to turn coloring on
or off), color.grep.match (to set the color of matches), --color
and --no-color (to turn coloring on or off, respectively).

The output of external greps is not changed.

This patch is based on earlier ones by Nguyễn Thái Ngọc Duy and
Thiago Alves.

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>