This introduces some important new options for Spina, designed to fix

problems encountered as we ported more class hierarchies from Backbone

to Spina.
ES6 classes have static attributes (which must be accessed on the class

constructor name) and instance attributes (which are only created when a

class is instantiated). What's missing are attributes that live on the

protocol (more akin to Python class attributes), which are what Backbone

really expects.
The recommendation is now to use static attributes. Spina now provides a

prototypeAttrs option for classes that are used to specify static

attributes (like events, defaults,tagName`, etc.) that must be made

available on the prototype. On class preparation, getters/setters will

be injected on the prototype to wrap the static attributes.
To aid in keeping attributes manageable, there's also a new

automergedAttrs option that can be defined on a class. This is used by

the class and any subclasses to merge together attributes containing

keys. For example, if a parent class and subclass both define events,

and this is in some base class's automergedAttrs, then the two will be

combined, reducing the chances of missing entries.
skipAutomergedAttrs can be used by a subclass to turn this behavior

off for select attributes, or all attributes.
These will be enabled soon for the built-in classes, and consumers can

use these to help transition to static-only attributes and to refine

class architectures.

Tested these new options in Review Board when converting classes to

Spina. Verified that specified attributes would merge and were

accessible on the prototype.