bug#6227: Color isearch regexp submatches differently

[Drew Adams]

> > Did you mean something like this (attached)?
> > This is how I highlight submatches in Icicles search.
> >
> > (The top part of the image, with light blue background, 
> > shows the highlighting. The bottom part of the image,
> > with white background, shows the regexp used and
> > is just an explanation of the subgroup highlighting.)
> 
> Yes, exactly.

IMO, this can be very helpful when searching with regexps.
And it can help users learn about using regexps more generally.

Of course, such highlighting should be optional, and preferably via a toggle
during Isearch.