grep: allow -E and -n to be turned on by default via configuration

grep: allow -E and -n to be turned on by default via configuration

Add two configration variables grep.extendedRegexp and grep.lineNumbers to
allow the user to skip typing -E and -n on the command line, respectively.

Scripts that are meant to be used by random users and/or in random
repositories now have use -G and/or --no-line-number options as
appropriately to override the settings in the repository or user's
~/.gitconfig settings. Just because the script didn't say "git grep -n" no
longer guarantees that the output from the command will not have line
numbers.

Signed-off-by: Joe Ratterman <jratt0@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>