Recursively describes the includes of array schemas #312

Fixes #312

* Walks the rules array to find includes
* Describes each included type
* Pushes the descriptions onto a new `.includes` property of the description.

There doesn't currently seem to be a good way to get at just the includes. I could either dig into this._test or the existing description rules. I chose the latter since it felt marginally cleaner. The alternative is to track includes in the Array type, but that was a bigger change and I wanted to presume as little as possible. It's simple enough to simplify this logic to use a this._includes if that's preferred.

Similarly, I'm not currently doing this for excludes, but if that seems like good behavior it's simple enough to add.

I have some (small) stake in this from #241, as I'd need to get into the include types of arrays as well for metadata in some cases. I thought I'd try to be helpful in the meantime.