Feature request: give access to the root injector

[ukstv]

It would be nice to have access to the root injector via injector.root to later call dispose() on it.

Now I have to maintain two references to achieve disposability i.e. root.dispose():

1. Root injector: const root = createInjector()
2. Fully populated injector: const injector = root.provideClass('foo', Foo).provideClass('blah', Blah)

It would be cool if I could maintain just a single reference to later do this:

const injector = createInjector().provideClass('foo', Foo).provideClass('blah', Blah)
injector.root.dispose()

[nicojs]

This sounds reasonable. I'm open for a PR.