This introduces an extend() method on Spina classes that behaves like

the Backbone extend() methods. This can take prototype properties and

static properties and will construct a new Spina-compatible subclass

that behaves just like an older-style prototype-based Backbone subclass.
With this, base classes can be converted incrementally to ES6 classes,

without breaking any legacy subclasses or altering any behavior.
The implementation is pretty straightforward. The biggest problems to

solve weren't class construction, but rather the differences between

legacy classes (which use prototype-based attributes) and ES6 classes

(which have static attributes or instance attributes, but no prototype

attributes). Those issues have been solved by the recent option changes,

allowing extend() to be able to work.
This also introduces a __super__ attribute on instances of the

classes, which is needed for compatibility with Backbone's __super__

attribute. This can help properly determine parent classes for legacy

class instances.

Verified the Review Board test suite passed when using this along with

other Spina 2 changes and converting some central base classes to

ES6 classes.