Implement javaEnums extension

[ddcruver]

Implement javaEnums extension to extend on what javaEnumNames provided.

[ddcruver]

@joelittlejohn I have additionally changed some of the methods to protected to allow a custom EnumRule to be able to extend the rule in multiple different discrete ways instead of copy-n-pasting or reimplementing it all.

For example in a custom EnumRule I would like to write for my project I would like to even override the Constructor that is generated and add additional fields to the enum. The actual use case is obscure so I am implementing it outside of this project.

[ddcruver]

@joelittlejohn can we get this pull request in?

[joelittlejohn]

If you could add something to https://github.com/joelittlejohn/jsonschema2pojo/wiki/Reference#extensions that would be great 👍

[ddcruver]

I will put it on my todo list.