Set a default processor for CliOutput objects

As reported at https://github.com/joomla/joomla-framework/issues/354 CLI apps must set a `ProcessorInterface` object to a `CliOutput` instance for the application to be usable.  To fix this, I set a default instance in the `CliOutput` constructor if a `ProcessorInterface` isn't passed in on instantiation and changed the `getProcessor` method to throw an exception (which shouldn't be necessary now) if an object isn't set.

If anyone cares to look, I'm getting an unexplained test failure in my tests for `Stdout` in that it seems like a processor object can't be injected via the constructor (it's always returning null for me).