Pass in coupler directly

[SomeKittens]

instead of:

gustav.addCoupler(new GustavRedis());

just pass the coupler directly as a source:

let red = new GustavRedis();

gustav.createWorkflow('asdf')
  .source(red, 'some-channel')
  // ...

Now it's easier to work with different types couplers, and has fewer magic strings from default names.

[SomeKittens]

+1, I'm leaning further away from magic strings and globals hidden in Gustav.