color-words: Support diff.wordregex config option

color-words: Support diff.wordregex config option

When diff is invoked with --color-words (w/o =regex), use the regular
expression the user has configured as diff.wordregex.

diff drivers configured via attributes take precedence over the
diff.wordregex-words setting.  If the user wants to change them, they have
their own configuration variables.

Signed-off-by: Boyd Stephen Smith Jr <bss@iguanasuicide.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>