Allow all deepmerge.Options to be set

[ryleyb]

This is just a proof of concept to cover issues with custom object types being converted to plain objects. See
https://www.npmjs.com/package/deepmerge#ismergeableobject for how they describe the issue.

Using this fixes #216 (see example in README.md).

I have never used TS before, so I'm sure I've done some things wrong, I just wanted to show how it could be fixed.

[ryleyb]

To be clear - the point of this (for me) is that in my reducer I want to call methods of my classes to change the state from an instance of my class, to some sort of primitive, and then in restoreState re-inflate that data into my class. Without this change, in reducer the classes have already been crushed down into plain Objects, so I can't do anything with them.