stddoclet: Tag listener interface

Tag listener interface - This would make it simple to add
new tags that Javadoc doesn't know about. All you would
have to do is write a listener class that tells how to
deal with the new tag. (Suggested by Steve Lewis, who
gave the Warnlet/Codelet BOF.)

56% of developers at '99 JavaOne Javadoc Bird of a Feather
voted for this -- the most highly rated request.