[WIP] Adds htmlbars `render` helper

- [x] make tests pass for simple scenarios, like "{{render 'foo'}}"
- [ ] make tests pass for passed-in controllers, like "{{render 'foo' controller='bar'}}"
- [ ] make tests pass for passed-in context, like "{{render 'foo' baz}}"
The `{{outlet}}` helper just landed (which added this), so you will likely need to remove this on rebase.