Add support for off-set terms in GLMs and NegBin methods

[jthaman]

[karlindberg]

Hi! I'm a big fan of the ciTools-package and I'm currently trying to use it to get prediction intervals for a NegBin-regression which uses an offset. I understand that this has not been implemented yet - do you know of a manual workaround for this (to get the intervals) and/or another package with this feature?