add feature to choose the property name of the primary key of collection - #159

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I signed it!

Arf maybe I should create my merge request on more-dev branch instead of master..

I think this is a good idea. It's an extension I've wanted myself and had intended to do something like this and along the lines of the id handling in @ngrx/entity.

I don't know when or if it will happen. I'm keeping your PR around as a reminder of what could be.

You may want to fork this library for your own needs.