Add exception-class-based parameter value scrubbing

While it does convert the original exception to a `FlattenException`, I think this is a valuable addition. Unfortunately I couldn't find a way to do it earlier, outside of this bundle due to the private and final usage throughout the SPL Exception class, and the typehinting of that class through the response eventing chain. 

The configuration for this looks like so:

```yaml
staffim_rollbar:
    scrub:
        exceptions:
            - PDOException
        parameters:
            key: %key%
```

The values are matched fairly loosely. `array_walk_recursive` looks for string values from `scrub.parameters` which are then replaced by the key. That also means you can describe your values, if you wish.