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It wasn't a reliable way to determine if the command was "done". We need to wait
for run, post-run, etc. to finish first.

* Fixes #143

This commit:

1. Removes the exit channel from the CLI response emitter.
2. Adds a `Status()` function to the `cli.ResponseEmitter` as a replacement
   for (1).
3. Renames `Exit(code)` to `SetStatus(code)`. This new function doesn't
   immediately _do_ anything other than set the status.
4. Make sure to wait for _everything_ to finish before returning from `cli.Run`.

We *always* expect environments to be able to return a context so we might as
well enforce this with the type system.

They depend on *this* package anyways.