Automatically close unclosed HTML tags.

Fixes an old problem with some unclosed <b> or <div> messing up
the layout of docs.

In addition to automatically closing the tags, a warning gets printed.
Extract HtmlStack class from DocFormatter.
Implement --ignore-html option.

Allows for ignoring additional HTML tags which otherwise would get
closed down by the new automatic tag closing mechanism.

Useful for ignoring ExtJS preprocessor directives: <debug>, <locale>.