Add iFrame to idom.html

[Archmonger]

Current Situation

Currently, iframes are missing from our standard VDOM types. Right now users are forced to manually create iframes via make_vdom_constructor.

Proposed Changes

Add iframe to idom.html

Implementation Details

iframe = make_vdom_constructor("iframe")

[rmorshea]

For now the best option is just to use vdom("iframe", ...) unless you want a dedicated function for that.