Add the --color-words option to the diff options family

With this option, the changed words are shown inline. For example,
if a file containing "This is foo" is changed to "This is bar", the diff
will now show "This is " in plain text, "foo" in red, and "bar" in green.

Signed-off-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Junio C Hamano <junkio@cox.net>

diff.h

@@ -46,7 +46,8 @@ struct diff_options {
 		 full_index:1,
 		 silent_on_remove:1,
 		 find_copies_harder:1,
-		 color_diff:1;
+		 color_diff:1,
+		 color_diff_words:1;
 	int context;
 	int break_opt;
 	int detect_rename;