Major Release: ability to merge nested/deep objects, increased test coverage, and minor props change

New Features:

• the ability to merge a nested/deep object
  • here's an example / the test case: https://github.com/dgreene1/merge-partially/pull/92/files#diff-6b623d6bcf5e7f06c466aa060ec9c4b6R132
• exposes the NestedPartial type as a convenience so you can explicitly type the override object. Example here: https://github.com/dgreene1/merge-partially/pull/93/files#diff-04c6e90faac2675aa89e2176d2eec7d8R70

Breaking Changes:

• I really couldn't come up with a reason why someone would want to avoid passing in an override object. So the second parameter is no longer optional. I would be very surprised if this breaks anyone. But I made this a major version release anyway.