Rewrite HTMLFilter interface to use HTML::Tree

[gflohr]

The HTMLFilter interface currently runs HTML::Parser over the document and lets the plug-ins install handlers for the various events. That is not flexible enough.

It's better to use HTML::Tree so that plug-ins have a chance to evaluate context.

Incompatible change that is unlikely to break existing sites.

[gflohr]

Probably the better option would be to try interfacing with https://cheerio.js.org/ so that you can use jQuery style DOM manipulation.