Add new_only option to allow notify on all events

Currently the plugin only notifies on new events, but it would be useful for it to notify on all events. This PR, inspired by [sentry_hipchat](https://github.com/linovia/sentry-hipchat/blob/master/sentry_hipchat/models.py#L74), adds a `BooleanField` of `new_only` that makes the behavior optional.

I haven't exactly written a plugin before and haven't had time to test this, but it looked simple enough that I thought I'd do a PR to get things started.