allow callables to set filter level for exceptions

this means that applications can be easily configured to change the status (ignore, warning) of exceptions based on things that matter to them.

e.g. maybe there are some paths that you just don't care about ActionController::RoutingErrors for
e.g. maybe errors that happen to certain user classes can be marked as warning rather than error

see: https://github.com/rollbar/rollbar-gem/issues/61