Pluggable normalization

[stubailo]

The goal is to allow people to pass their own ID getter functions to pick the key that is used for an object when it is put into the store. This addresses #204, but also makes #180 much easier.

TODO:

• Update CHANGELOG.md with your change
• Make sure all of the significant new logic is covered by tests
• Rebase your changes on master so that they can be merged easily
• Make sure all tests and linter rules pass

Testing strategy:

• Check internal store of apollo client when option is passed to make sure it is used when writing to store; internal functions are already tested