Added typesafety for multiple tags

[JaapWijnen]

Hey! Really like the library. I was playing around with it and saw that it was possible to do something like

.document(
  .html(
    .body(
      .html()
    )
  )
)

And tags like .div and .script can be used anywhere in .document.
.script can also be used anywhere within .html which are all examples of invalid html.
I was playing around a bit and it would be quite easy to improve on this behavior by

• introducing an extension on ChildOf for doctype.
• remove general extensions from ChildOf for script tags (and some others) and move these instead to specific extensions of ChildOf (introducing a little bit more code)
• move all tags that are only used inside a body tag to an extension of ChildOf where Element == Tag.Body All these static functions would also return a ChildOf<Tag.Body> instead of the previous Node type. Then we can use them nested as well!

I've got this working locally and works quite nice! :) Let me know if you're interested in a PR for this behavior!

[stephencelis]

I'm going to convert this to a discussion. I think we're always open to more safety. Just have to be mindful of things that affect compile times :)