Migrate back to html2text.

Migrate back to html2text.

I gave up on using it previously because it lacked some important
features - in particular, it didn't support CSS on <a> elements, which
is necessary for identifying hashtags and mentions in Mastodon's
output HTML.

In 0.9.2 that feature is present, so I can try again! Also, 0.9.2 has
the neat ability to save the RenderTree that's an intermediate product
of the pipeline, just before we depend on the width. So we can keep
that after parsing a toot, and then only re-run the necessary back end
of the pipeline to re-render at a different width.

The amount of code is extremely reduced.