Enhancement: Enum Support

[DylanYoung]

Requires to #45

I'd like to see Enums supported for Definitions. Not sure on the design yet (perhaps enum-eligible fields could be specified by json path?), but would love to hear your thoughts!

[wolverdude]

This pending version (1.2) will open up the schema builder for extension, so this should be easy to implement as an add-on, and it could be activated through a seed schema.

I'd also be interested in implementing this (and potentially several other common keywords) as part of the base functionality in the next version after this.

[wolverdude]

Current status, on hold. Initial implementation ideas are here.