change to 'subst'

[Mike T. Miller]

I'd like to add "subgroup" awareness to 'subst' so that things like

  RE re = new RE("(xyz)");
  System.out.println(re.subst("abcxyzabc", "-\\1-"));

will produce "abc-xyz-abc".

This wouldn't take me much to put in.  Is there any objections?

(Maybe the subgroup character should be '$' instead of '\'?  I've
seen it both ways.)

Mike