add --no-tags argument to list command

[beuerle]

This argument works exactly like --no-docs and only lists publications
without tags, fixes #237.

Currently publications without a tag have an empty set as the tags
variable. If this changes in the future (see #234) the conditional has
to be updated.

[benureau]

Thanks! With each new functionality, we need even a small test to ensure coverage and verify that it does function appropriately across different versions and OS. We need to improve how easy it is to write test and the documentation for them. If you don't know how to do it, I will create a test for that PR.