Enable multiple modules per plugin

[ymauray]

This pull request allows a plugin to expose multiple modules, therefore multiple keys/actions in one single plugin. It is backward compatible with existing plugins.

Example :

func GetModule1() handlers.Module {
        return handlers.Module{
                ....
                Name: "Module 1",
        }
}

func GetModule2() handlers.Module {
        return handlers.Module{
                ....
                Name: "Module 2",
        }
}

func GetModules() []handlers.Module {
        return []handlers.Module{GetModule1(), GetModule2()}
}