RFC: Engines

[RFC](https://github.com/tomdale/rfcs/blob/master/active/0000-engines.md)
FYI: `document.createElement("foo:bar")` works (holdover from XML days, I suppose).

I think our solutions are going to end up being:

* Template-wide imports (`*` or named)
* Scoped imports (like XML namespaces or what Stef suggested)
* Changes to the default namespace (would require `html:div` if HTML was desired).

I think that many solutions will work, and that as long as we have a clear, "one true story" for doing this, and it doesn't introduce excessive duplication or boilerplate, people will be able to live with it.