Add Netssh#stream to give real-time access to stdout & stderr

The main reason for this is so that you can print STDOUT as it is received.

A contrived example:

```ruby
on "root@example.com" do
  cmd = 'for i in $(seq 20); do echo "the time is $(date +%H:%M:%S)"; sleep 1; done'

  stream(cmd) do |type, data|
    print data
  end
end
```