Add a way to change the project config we want to load for a specific project when a command is started

[StephaneCapponi]

Today each project contain a config.py file that is used to store variable. The user can change this file by redefining the settings_module of the ProjectConfig. However if the user define mulitple config file, there is no way yet to select which want to use at runtime.

It would be great to add a --config argument to every ProjectCommand in order to select the config we want to use.