add per-event-type support to add_subscriber_predicate

[mmerickel]

As described in #1545 (comment) we should add event= to the config.add_subscriber_predicate API to allow a different predicate per-event-type. This can be done with a simple adapter. We may be able to then register some default predicates for the builtin events.

Note the biggest issue is that BeforeRender exposes the request as event['request'] instead of event.request which is a big reason an adapter-based api is necessary, but also because events have no required format.

[mmerickel]

The alternative of course is to simply handle the various event types inside each individual predicate instead of registering adapters.

class ContextPredicate(object):
    def __init__(self, val, config):
        self.val = val

    def __call__(self, event):
        if isinstance(event, BeforeRender):
            request = event['request']
        else:
            request = event.request

This is what you currently have to do with no changes, and personally I find to be reasonable. I just opened this issue incase someone was offended by it and was looking for a way to improve things similar to the efforts in #1545.