Add a command print_named_configs

[thequilo]

I found this command very useful, especially when an experiment contains multiple ingredients which define (nested) named configs. This refers to #239 and may be used to print the available named configs when the user requests an unknown config.

This command is added to the main experiment and prints a list of available named configurations. Docstrings for config scopes are copied and printed as additional information for the named config colored in grey. Here's an example of what the output looks like:

Named Configurations (doc):
  named_config1
  ingredient2.named_config3
    """
      A docstring with
      mutliple

      lines.

      """
  ingredient2.dict_config
  ingredient1.named_config2   # A beatiful docstring (this is colored in grey)

Btw: This is my first pull request so please correct me if I am doing something wrong.

[coveralls]

Coverage increased (+0.09%) to 83.516% when pulling 3b88c53 on thequilo:print-named-configs into 3052552 on IDSIA:master.

[Qwlouse]

Wow, that is a very nice PR and a useful feature indeed. It will also come in handy for #239. Thanks a lot for your contribution!

[thequilo]

Thanks :)