Let user enter credentials via command line

[StegSchreck]

possible ways:

• as argument
• prompt for it when needed
• prompt for all needed credentials before starting

[StegSchreck]

• as argument is not feasible when using multiple destinations
• prompt when needed is not comfortable, when starting the script and letting it run (as it might take a while), especially when using multiple destinations.

This leaves the prompt at startup of the script for all sources and destinations used later.

Additional thought: think about re-prompting for when login failed.