Compose middleware resolve conflicts with #393

[seratch]

Summary

This pull request fixes git conflicts with slackapi#393 .

Two test suites I've added detected an issue where all `app.event` listeners are not called in this branch. I'm still thinking of a proper way to fix it. Any thoughts?

  91 passing (8s)
  2 failing

  1) App
       event processing
         middleware and listener arguments
           logger
             should be available in middleware/listener args:

      AssertionError: expected false to be true
      + expected - actual

      -false
      +true
      
      at Context.it (src/App.spec.ts:694:18)

  2) App
       event processing
         middleware and listener arguments
           client
             should be available in middleware/listener args:
     TypeError: Cannot read property 'token' of undefined
      at Context.it (src/App.spec.ts:760:35)

[seratch]

The test failures were just my bad... 🙏 I've fixed them.

[barlock]

Thanks for doing the merge for me!