[Fix #152] Add `AdditionalMethods` configuration to `Minitest/GlobalExpectations`

[tejasbubane]

Closes #152

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• [x] Wrote good commit messages.
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• [x] Added tests.
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[tejasbubane]

@koic can you review this please?

[koic]

Can you update the parameter name to ExtensionMatchers and mention it in the documentation text?

[tejasbubane]

@koic Changed to ExtensionMatchers.

[tejasbubane]

@koic ping

[koic]

Hm, the approach we should take will depend on what the custom matchers are like. I've asked the issue author @gi the use case. Let's wait for a reply. https://github.com/rubocop/rubocop-minitest/issues/152#issuecomment-997421408

[koic]

I'm going to close this PR because no response for a long time from #152. Flexibility without really necessary extensibility introduces unnecessary complexity at the cost of simplicity.