Allow control of local stdout/stderr

[bitprophet]

Feature request: even in pty=False, combine_stderr=False situations where we have access to both remote stdout/stderr, it's sometimes desirable to change what local streams those go to.

E.g. wanting to combine both stdout and stderr into local stdout, while still having access to distinct .stdout and .stderr results from the run() statement. (Otherwise, using combine_stderr would be the right solution.)

This can and probably should extend to allowing user control over what stream objects Fabric writes the remote streams to, in case they want to use their own eg StringIO objects.