Add a --dry-run option to the update command

[jaydorsey]

Maybe a command to see what the changes would look like?

ec update heroku-app-name1 heroku-app-name2 --key=MY_KEY --value=asdf --dry-run

It would show you which changes it would make when it runs

             env | current value | new value | action
-------------------------------------------------------
heroku-app-name1 | FOO           | asdf      | updated
heroku-app-name2 | <nil>         | asdf      | added

This could also maybe just run when the updates are actually made, so you can see the outcome? (maybe separate issue/PR)

[smridge]

Ooo this would be nice!