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CloseNotifier has been deprecated for a while.

Also, ditch the "environment" context. We don't actually _need_ this.

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`.