Support for Element::add_attribute #205

I'm using the ruby binding for lol_html and would really like to be able to transform <p>Foo</p> into <p data-this-is-special>Foo</p>.

I understand that the lol_html API currently implements Element.html#set_attribute with a String value, but does not yet provide a similar Element#add_attribute-method which writes an attribute without a String value.

Is Element#add_attribute something other ppl are interested in and something someone else would be willing to implement? I myself have no rust knowledge (yet) and would have to depend on someone else to provide the code here :)