README recommends placing secrets in plain text on the command line

[jdufresne]

In the "Config command" section https://github.com/airbrake/airbrake-cli#config-command the following snippet exists:

airbrake config set user-key YOUR_USER_KEY_HERE

This suggests the user put their secret (user-key) on the command line in plain text. This is normally consider a bad practice in security realms as:

1. Unix commands like ps and top often show command line arguments to other users, thus leaking the secret
2. The command gets place in the shell history, thus leaking the secret

It would be nice if the secret could be set either:

1. Through an environment variable
2. Command stdin/stdout piping