Checklist

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What problem does this solve?

In golang v1.13 the notion of error wrapping was introduced with the following optional method signature on error types: func (*T) Unwrap error. The ExitCoder type is very similar to a wrapped error with addition information (the exit code), so it seems like a good addition.

A long term consideration (for this package's v3) might be to change the implementation of ExitCoder to be:

type exitError struct {
	exitCode int
	err      error
}