Read input from file when starting argument with @

[ajuvercr]

When configuration is asked, a nice feature to have would be to start the input with @ then nautirust will read this file and use it as argument.
This can either be in generate or in run. run would make more sense so the user can dynamically update config in his/hers/theirs own config file and not in the generated config file.

This can also be used with channel configs, which would be much better than duplicating the config which happens now.