Output level alias for stdout + running #506

When cleaning up some moderately verbose (in terms of the output) fabfile code, I find myself often wanting to hide stdout and running at the same time -- but not stderr or anything else. In other words, the Unix "silent unless something is wrong" convention. (Though actually wrapped in a "Doing xyz ... done" context manager; see #505 for that.)

I think a good name for this would be 'commands', as it's hiding "the command" but not anything unexpected. (Better would be to rework 'running', but that's not backwards compatible so hands tied.)