DefaultSession can be embedded in all mixins and sub-command handlers and provide a default implementation of the Session interface which uses os.Stdout, os.Stderr, and os.Stdin by default.

Input defines the framework-agnostic user input passed to handler's Run methods.

PostRun is optionally implemented by handlers/mixins to perform tasks after Handler.Run finishes.

PreRun is optionally implemented by handlers/mixins to perform tasks after flags are parsed but before Handler.Run.

Session defines CLI handler behaviors which are useful to replace with test doubles.