Add Null Keyword

[WoH]

Goals

1. Support TypeScript's null keyword
2. Represent null as a "nullable" enum with type [null]
3. Distinguish between nullable (null) and required/optional (undefined)
4. Support Swagger whenever possible*

* OpenAPI can use anyOf, to combine schemas. In case of Swagger, we need to check if we are combining a non-$ref type with null. In this case we can set x-nullable: true. Otherwise, we need to bail out since {$ref: '', ['x-nullable']: true} does not work/will be ignored (sibling values along $ref are ignored).

All Submissions:

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• Have you written unit tests?
• Have you written unit tests that cover the negative cases (i.e.: if bad data is submitted, does the library respond properly)?
• This PR is associated with an existing issue?

Closing issues

Closes #557, #479, #497 (presumably), #257 (presumably), #510

If this is a new feature submission:

• Has the issue had a maintainer respond to the issue and clarify that the feature is something that aligns with the goals and philosophy of the project?

Potential Problems With The Approach

Breaks compat

[dgreene1]

Looks good to me! Great work! :)

[WoH]

I'll try to add more tests around validation and integration, but I wanted to make sure we are all on board with the approach.