Allow '=' and '~' in unquoted attribute values.

Added overridable methods handle_starttag(tag, method, attrs) and
handle_endtag(tag, method) so subclasses can decide whether they
really want to call the method (e.g. when suppressing some portion of
the document).

Added support for a number of SGML shortcuts:

        shorthand               full notation
        <tag>...<>...           <tag>...<tag>...
        <tag>...</>             <tag>...</tag>
        <tag/.../               <tag>...</tag>
        <tag1<tag2>             <tag1><tag2>
        </tag1</tag2>           </tag1></tag2>
        </tag1<tag2>            </tag1><tag2>

This required factoring out some common actions and rationalizing the
interface to parse_endtag(), so as to make the code more readable.

Fixed syntax for &entity and &#char references so the trailing
semicolon is optional; removed explicit support for trailing period
(which was a TBL mistake in HTML 0.0).

Generalized the test program.

Tried to speed things up a little.  (More to come after the profile
results are in.)

Fix error recovery: call the end methods popped from the stack instead
of the one that triggers.  (Plus some complications because of the way
HTML extensions are handled in Grail.)