Add auto-generated types for valid HTML descendants

[willmartian]

Currently, hdot does not type-check the content category of children passed to other elements. This feature needs to be added to the element types here.

The following should cause a type error, since <link> is an empty element.

h.link('Hello world'); 

Find more info here: https://developer.mozilla.org/en-US/docs/Web/Guide/HTML/Content_categories