Sketch out support for matplotlibrc-like config dictionaries

@tacaswell I ended up moving `RcParamNG` to `MPLConfig` just for kicks. Other than that, the only substantive changes is the `from_user_config` method and a fix for old-style classes (`contour.ContourSet` broke the `type` check).

The rest of the code sits alongside yours and adds parsing for dictionaries that look similar to what you might get out of matplotlibrc files. Right now, only 'lines.linewidth' is connected.