New base class for the SGMLParser and HTMLParser classes from the sgmllib

and HTMLParser modules (and indirectly for the htmllib.HTMLParser class).

This has all the support for scanning over DOCTYPE declarations; it warrants
having a base class since this is a fair amount of tedious code (since it's
fairly strict), and should be in a separate module to avoid compiling many
REs that are not used (which would happen if this were placed in either then
sgmllib or HTMLParser module).