Setting a property on an object's prototype with function

[D4rk1ink]

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[D4rk1ink]

Please approve this PR.

[podefr]

thanks, I appreciate your contribution and I like the idea of using a function for setting a property.
It seems that your changes cover the following uses cases:

1. Setting a property at the root of the object set(obj, 'a', (value) => value * 2)
2. Setting a nested property set(obj, 'a.b.c', (value) => value * 2)
3. Setting the current object's prototype (not Object.prototype).

I think that your tests only verify that 2 and 3 work, but not 1. i.e we don't test this line: https://github.com/cosmosio/nested-property/pull/17/files#diff-e727e4bdf3657fd1d798edcd6b099d6e092f8573cba266154583a746bba0f346R127 (127)

would you be able to add the appropriate test?
would you also be able to edit the README's documentation to show the new feature?

Thanks!