Justanshulsharma feature/encrypted data flag

@justanshulsharma Thanks again for the work you did on adding this feature! After some discussion we decided to modify the behavior somewhat, so I've added work to your PR here.

@serdar I don't have any more time to work on this before I leave. The code and unit tests for show and edit still need to be added. Also, I don't know if we have checks in place for what happens if knife.rb specifies `secret` and `secret_file` or if we need to add those to the code here.