color-words: take an optional regular expression describing words

color-words: take an optional regular expression describing words

In some applications, words are not delimited by white space.  To
allow for that, you can specify a regular expression describing
what makes a word with

	git diff --color-words='[A-Za-z0-9]+'

Note that words cannot contain newline characters.

As suggested by Thomas Rast, the words are the exact matches of the
regular expression.

Note that a regular expression beginning with a '^' will match only
a word at the beginning of the hunk, not a word at the beginning of
a line, and is probably not what you want.

This commit contains a quoting fix by Thomas Rast.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>