Support for custom migration resolvers

[markrileybot]

Summary

Helps with feature: https://github.com/hhandoko/cassandra-migration/issues/82

Adds support for custom migration resolvers. It seems that the support is planned based on the commented fields in the configuration interface. Anyway, this would really help me out as I have some crazy things I have to do and a custom resolver would work just fine.

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Pull Request (PR) Checklist

Documentation

• [ ] Documentation in README.md or Wiki updated
• [ ] Update Release Notes if applicable -- collaborator-access only

Code Review

• [x] Self code review -- take another pass through the changes yourself
• [ ] Completed all relevant TODOs, or call them out in the PR comments

Tests

• [x] All tests passes

[markrileybot]

Hi @hhandoko , Have you had a chance to review this PR?
Thanks!