Request to define all variables before use and check for undefined variables

[calebgcooper]

This is more of a feature request than an issue. This library is fantastic and I've used it for many years.

I like to run scripts with the so called bash strict mode:
set -euo pipefail

This helps to avoid unexpended errors due to undefined variables etc.

When I try and run a script leveraging bash concurrency I cannot set -u because of undefined variables.

Would it be possible to define defaults for CONCURRENT_DRY_RUN and to check if remaining_args is undefined.