Basic boolean attribute support

`attribute="true"` is not strictly supported for boolean attributes.
They should be `attribute="attribute"` or just `attribute`

This commit makes anything that is TrueClass come out as
`attribute="attribute"`, and any `FalseClass` is ignored.

http://www.w3.org/TR/html4/intro/sgmltut.html#h-3.3.4.2