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This is a breaking change and will affect Filecoin. However, filecoin is
currently using this incorrectly anyways (breaking PreRun and PostRun).

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`.

This time, we completely separate error encoding and let the emitter deal with it. Basically:

1. Some transports won't even use the same channel.
2. We don't *have* an error "encoder" for protobuf.

We now *finally* return the correct content type (json) when returning an error after passing `--enc=protobuf`.

This *also*:

1. Removes GetDecoder. We need to special case this every time anyways.
2. Allows transports to specify the "default" encoding type instead of assuming JSON.
3. Ensure that we never mistake a value for an error in the HTTP transport by converting to a cmdkit.Error *before* passing errors to `doPreamble`.

They depend on *this* package anyways.