Support ProMotion stub generation.

I'll be working on the [ProMotion](https://github.com/clearsightstudio/ProMotion) side of things here in a bit to add support for mixing the `ib` and` ProMotion` gems, but I wanted to get this into the gem so that it can properly do the ProMotion integration since every ProMotion `Screen` inherits from `UIViewController`.

One thing I noticed is that the `super_class` object should be something like `PM::Screen` or `ProMotion::WebScreen` however, the `ib` gem only sees these as `PM` and `ProMotion` respectively. Just an observation.

cc/ @jamonholmgren