Implement different list/map merge strategies

[nightkr]

Fixes #128.

[Arnavion]

I made some changes and pushed to https://github.com/Arnavion/k8s-openapi/compare/pr-138

• Moved MergeType to be in the Schema itself instead of the three kubernetes_* fields
• For the merge_strategies, renamed old to current
• Changed the codegen for the strategies that take callbacks to be over multiple lines for readability.
• Fixed the CRD tests.
• Misc formatting changes.

Does it look good to you?

Also @soenkeliebau FYI.

[nightkr]

Looks good to me, thanks! Sorry about the very slow reply.

[nightkr]

Thanks!