Add `get_namespace` method on `Config` object for convenient access of namespaced config options

Here's a little proposal for a new function on the `Config` object. I often find it very useful to grab namespaced config options. I've used my own helper function for ages now and thought it might be useful to everyone. It's particularly useful for me when configuration options map to a set of keyword arguments for a function or a class constructor. A  test has been added as well to verify the feature.