Add support for additionalProperties with Bool value

[mossprescott]

According to the spec (here), additionalProperties can be true (meaning additional properties with any values) or false (meaning the same thing as the attribute not being present, I believe) or a schema.

This commit adds a new type, AdditionalProperties, with custom JSON encoding to deal with that, and what appears to be reasonable validation for it.

I did not write a test that validation actually does anything sensible with this new option, because it wasn't clear to me where that would go, but I'd be happy to look into it if the basic approach taken here seems reasonable.

[fizruk]

@mossprescott this is great!

I've reread the spec and you appear to be correct. Thank you for noticing this tiny bit in the spec :)

The code looks fine. Let me know if you want it merged and released rightaway, or if you want to work more on tests!

[mossprescott]

I'm comfortable with it as is; I just added a simple validation test to the branch, and I've been using it in our project and it seems to do what we need.

I'm not 100% confident that validation is correct if you use additionalProperties: true in combination with other properties, but since we're not using that I'm prepared to trust it since you've looked at the code.

Note: I'm actually using a branch from 2.2.1 for the time being since we're on an earlier LTS, but it would be great to get this released so we can clean that up in the near future.

[fizruk]

@mossprescott it took me a mere month, but swagger-2.3 is out :)

Your change also allowed me to introduce ToSchema instance for aeson's Object in d72466a!