Streaming logs and IO streams to the client

Ideally we'd send logs and IO stream side effects over the RemoteREPL channel as they happen rather than buffering them in the server until the command is done executing. This would be especially helpful for long-running commands.

As part of this, it would be helpful if `@remote(stdout)` would forward a more faithful stdout-like object to the server so that `redirect_stdout` would work, making it easier to get the desired client-centric behavior mentioned in #29:

```julia
redirect_stdout(@remote(stdout)) do
println("hi")
end
```

Completely stateful redirection such as

```julia
redirect_stdout(@remote(stdout))
```

would also be helpful as it should solve part of #15