Support calling the new-mnemonic command in a non-interactive way

[remyroy]

It's not possible to use the --non_interactive flag with the new-mnemonic command. It wasn't design to be used that way. The flow for that command was designed to be quite interactive. I think it would be possible to adapt that flow to be used in a non-interactive way but first, I think we would need to figure out the use case for that.

I think the main use case would be for third party integration and/or automation. We would need to output the resulting mnemonic in a machine readable way for that third party or automation to be able to pick it up.

Related to #157