color-words: make regex configurable via attributes

Make the --color-words splitting regular expression configurable via
the diff driver's 'wordregex' attribute.  The user can then set the
driver on a file in .gitattributes.  If a regex is given on the
command line, it overrides the driver's setting.

We also provide built-in regexes for the languages that already had
funcname patterns, and add an appropriate diff driver entry for C/++.
(The patterns are designed to run UTF-8 sequences into a single chunk
to make sure they remain readable.)

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

userdiff.h

@@ -11,6 +11,7 @@ struct userdiff_driver {
 	const char *external;
 	int binary;
 	struct userdiff_funcname funcname;
+	const char *word_regex;
 	const char *textconv;
 };