help--browse: add '--config' option to check a config option for a browser.

help--browse: add '--config' option to check a config option for a browser.

The value of this new command line option will be used as a key to
check the configuration for an help browser.

This should remove the last bit in 'git-help--browse' that was
specific to 'git-help', so that other git command can use
'git-help--browse'.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>