Expose instantiated interpreter instance 

Currently cliff doesn't expose instantiated `InteractiveApp` instance to the `App`.

This means it's hard to "communicate" or call to the instated instance without doing something nasty like writing a custom `InteractiveApp` class which, for example, stores a references to instantiated object in a global scope / some kind of registry.

That is far from ideal so I propose assigning an instantiated `InteractiveApp` instance on the `App` instance instead.