format_attrs and _attr_decode as methods.
Refactor HTML.optimize_attrs().

Implement boolean attribute conversion.
Make HTML._attr_decode() a no-op.

The functionality has been absorbed into HTML.optimize_attrs().
Refactor 'format_attrs()' to 'HTML.render_attrs()'.
HTML constants.
'_bool' arg to 'HTMLBuilder.tag()'.
Changelog.
Delete TODO file.