Support multiple json config files.

[HebertS]

• Allow users to specify multiple json files with the -c options. Each specified json file needs to be separated by comma.
• The json file(s) will be searched in:
  1. The working directory
  2. The $RIPELIB/config path
• When there are multiple configurations for a given key, issue a warning and use the value defined in the first configuration file where it is described