Add the possibilty to load mulitple config file by project

[StephaneCapponi]

By default each project config load "management.config" module as a project configuration module. It would be interesting if we could set a list of config files to load. This would help organizing the project configuration files.

Something like:

class MyConfig(ProjectConfig):
    name = 'projects.myconfig'
    settings_module = [
        'management.config',
        'management.other'
    ]