Add documentation for emr-cli config file

[dacort]

Today, the EMR CLI supports a a config file for storing information related to your deployment environment (app ID, job role, etc), but it's completely undocumented. We should add:

• Documentation around the config file
• Command-line flags to save/update the config file, e.g. emr run would be much easier than emr run with 9 different flags. 😆 So we could add a new option like emr run --save-config that would save the set of arguments you used so you can reuse it the next time. This is useful when you're using the EMR CLI to iteratively develop/run a job in a remote environment.