feat: add ability to use custom zap core

[MDobak]

This PR adds the ability to replace the zap core with a custom one. It should solve the issue: #102.

I explained my reasoning for adding the SetPrimaryCore function in this comment: #102 (comment).

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[teghnet]

This is actually very useful.

[Stebalien]

This looks correct. And thanks for writing the test, it makes reviewing much easier.

[Stebalien]

Thanks!

[MDobak]

Thank you :)!