attempt to improve the system by which engine_from_config locates keys w/ special values
without resorting to just detecting ints, "True"/ "False" etc., if dialects could somehow register additional keywords to be used by the
_coerce_config() method, that would help a lot. I think just let them register their words and we kind of assume two dialects either don't use the same kw, or if they do, they have the same type. We're dealing mostly with booleans, should be OK.