Add an offset term to the Poisson family

[XianweiLi33]

Hi,
Thanks for the amazing package! It's super fast and deals with a wide range of data.
I was wondering if it's possible to add an offset option to the Poisson family, so that the package is not limited to simple poisson data but can also be used to deal with time-homogeneous Poisson processes.

Thank you very much!