prompt for credentials

[zaaarf]

Editor should prompt upon connecting to input any missing and mandatory config keys (right now only username and password)

This means the following:

• If neither username nor password are set in the config, prompt for both (in the same window, if possible).
• If only one of the two is set, the prompt should appear with the field already populated (if not possible to have them in the same window, skip one of the two?)
• If both are set, don't prompt and connect.