New function god-special-mode-p

This is a small refactor of the internal `god--special-mode-p` function
to make it available for general use.
Disable god-mode on comint-mode related buffers

A new predicate called `god-comint-mode-p' is provided and added to the
`god-exempt-predicates` by default. It returns t for buffers that
inherit from `comint-mode'.