Add the ability to mirror from a selected face

[blooop]

Hi,

Apologies if I have not some something correctly, this is my first pull request so still working out how to do things.

I added this feature after asking a question on in the cadquery github page:
#521

The featured adds support for mirroring about a selected face and adds an option to perform a union as part of the mirror operation. When mirroring about a selected face I think that in most cases a mirror union will be the desired operation rather than leaving two touching but unconnected bodies.

I have added some tests and updated the examples on how to mirror about a face. In the tests, I perform checks against the bounding box, but I think checking the volume would be better. I wasn't able to to work out how to call the method that returns the volume.

Please let me know if I have done anything incorrectly or need to add anything

Thanks,
Austin

[blooop]

I have looked at the conflict and do not understand what is wrong or how I can fix it.