Add ability to create a custom variable expander

[studgeek]

I think it would be very helpful to support custom variable expansion. So the user could provide the code that determines what the child objects are. This can be very helpful for complex objects.

The code could be provided in the same way as custom stringifiers, but the advantage of this over a custom stringifier is that you can follow the tree and continue to expand as needed.

Way back when I worked in Smalltalk custom inspectors were a very important tool and we had them for all of our framework objects (DB objects, RPC objects, etc).

[asmeurer]

That's a great idea. One could specify to expand SymPy objects along .args for instance. I think as an API a function that takes an object and returns a list of "child" objects, or None to fallback to the default, should work.