Provide an alternative to the `--api-key` flag when adding a new server nickname

[dbkegley]

Currently we can only supply a server api key to the add command using the -k flag. To prevent api keys from leaking through user's bash history, we should provide alternative methods of providing the api key.

It would be great if -k isn’t passed, we use getpass or similar to prompt for the API key. We'll also want to update our documentation to encourage users to go this route as a best-practice