Add HTMLElement properties/methods

[nwolverson]

Per #48.

I worked from the IDL in that comment, removing properties which are inherited from Element, and the event handlers per @garyb's comment. Additional omissions (because they seem hard/non-obvious) are

readonly attribute DOMStringMap dataset;
attribute HTMLMenuElement contextMenu;
readonly attribute CSSStyleDeclaration style;

[garyb]

Thanks very much!