Optionally disable 'nullable' values to follow Typescript convention.

[h]

Previously, apispec.ext.marshmallow accepted both undefined and null values for optional fields. This is different from the convention followed by Typescript, which uses only undefined values in preference over null.

See: https://github.com/Microsoft/TypeScript/wiki/Coding-guidelines#null-and-undefined

This commit adds an option called allow_null that enables the new functionality. By default, the previous convention is followed for backward compatibility.