Add support for specifying an additional config dir in the command line

[omry]

This is useful for standalone apps that want to come with their own configs but also allow users to add additional configs that can be used with the composition.

├── my_app.py
├── conf
│   ├── config.yaml
│   └── dataset
│       └── imagenet.yaml
└── user-dir
    └── dataset
        └── cifar10.yaml

config.yaml:

defaults:
  - dataset: ???

my_app.py:

@hydra.main(config_path="conf", config_name="config")
def my_app(cfg: DictConfig) -> None:
    print(cfg.pretty())

if __name__ == "__main__":
    my_app()

In the above example, normal run results in:

$ python my_app.py 
You must specify 'dataset', e.g, dataset=<OPTION>
Available options:
        imagenet

The app sees only the built in config dir conf.
The user can specify an additional config dir via the command line:

$ python my_app.py --config-dir user-dir
You must specify 'dataset', e.g, dataset=<OPTION>
Available options:
        cifar10
        imagenet

Now the app sees both configs.
The user now normally override the dataset:

$ python my_app.py --config-dir user-dir/ dataset=cifar10
dataset:
  name: cifar10