[Feature request] New contributors since a tag

[funnelfiasco]

Is your feature request related to a problem? Please describe.

In showing my work on #169 to coworkers, one suggested adding a similar query based on a tag instead of a time stamp. This would be helpful in calling out first-time contributors in a release announcement, for example.

Describe the solution you'd like

An option to show the contributors who made their first commit after the user-specified git tag.

Describe alternatives you've considered

I thought about extending my PR (#170) to cover this case, but since git tags are arbitrary strings, we could end up in a situation where the provided value is ambiguous.