Implement template registration, as discussed

[clarle]

Discussion of template registration in this gist: https://gist.github.com/caridy/5548076

I'll begin implementing this according to the spec that's been discussed for the past few months. It looks like most of it has been fully agreed with all around, but if anyone has any more ideas that they would like to bring up, please do so in this issue or in the gist above.

[clarle]

I've begun implementing this feature.

One of the discussions that came up while doing so is whether we actually need Template.unregister. Since the goal is to be able to load the templates in as modules, there's not really a situation where you would ever need to unregister a template. The original idea was that components that registered these templates would want to be able to unregister them before they were destroyed, but it doesn't seem like instance-level templates would ever be something that would be used.

For now, I'll be ignoring Template.unregister unless someone has another use case.

[clarle]

Merged in #1026.