Allow truthy script options.

Closes #288.

As per [spec](http://www.w3.org/TR/html-markup/syntax.html#syntax-attr-empty), the empty attribute syntax `<input disabled>` is exactly equivalent to specifying the empty string as the value for the attribute, as in `<input disabled="">`, I tried it in a few browsers and the behaviour is consist... I'll try later tonight in IE.

@matthewp done!