Try and resolve Extension classes from the DI container

[rossriley]

This is useful if the base extension has a complex dependency graph.

You can configure the base extension class in services.yaml eg:

Bolt\Extension\SomeCompany\CoolExtension:
        public: true

and it will then be constructed with dependencies resolved.

[bobdenotter]

Thanks, @rossriley

I've bumped Travis, and after it passes, I'll merge it in! :-)