Add --file argument to read data from a file

[wolf31o2]

This supports --env and --data, but is missing --file, leaving it up to the user to either serialize their data into a string or export it to the environment.

[mattsb42-aws]

If you would like to avoid duplication of effort, the AWS Encryption SDK CLI supports working with files directly.

http://aws-encryption-sdk-cli.readthedocs.io/en/latest/

[wolf31o2]

...and I found out that there's a hidden input in the JSON processing that defaults to sys.stdin but it's not available for single-value items.