Use setup.py entry_points to discover Custom Tags

[Sup3rGeo]

Hi,

I would like to propose using entry_points as another way of finding and loading custom tags.

This makes sharing and using tag implementations easier - avoids the need to specify them on every emrichen call (with -m mytag) and users can just pip install the package with the tag implementation (effectively becoming plugins)

If maintainers agree with the feature I would be happy to open a PR