Add documentation for emr-cli config file

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.

Start by locating the EMR CLI's existing config-file handling and the command definitions for `emr run`. Document the current config contents and usage, then determine how a `--save-config` option should update it and how saved arguments would be reused; done means the behavior and configuration workflow are documented and usable.

Written by the indexing model from the issue text.