Add option to disable commandline params for config file

[mathisloevenich]

Some commandline parameters should be disabled.

In my case I want to have a verbose mode, which should one be activated via the command line.

[tbooth]

I've proposed a way this could be implemented with a small change to the way parser subclassing works. See scenario 1 at:

https://github.com/tbooth/ConfigArgParse/blob/devel/tests/test_config_parser_subclasses.py