Sanitize extra data passed into sentry

The need for this arose through use of the celery integration, which [passes task arguments as extra data](https://github.com/getsentry/raven-python/blob/5.1.1/raven/contrib/celery/__init__.py#L38). The [SanitizePasswordsProcessor docstring](https://github.com/getsentry/raven-python/blob/5.1.1/raven/processors.py#L64]) implies this is expected behavior.
It's `data` because we need the argument to be passed by reference.

Works:
```python
>>> data = {'extra': {'password': '123456'}}
>>> def filter_extra(data):
...    data['extra'] = {'password': '*********'}
>>> filter_extra(data)
>>> print(data)
{'extra': {'password': '*********'}}
```

Doesn't work:

```python
>>> data = {'extra': {'password': '123456'}}
>>> def filter_extra(data):
...    data = {'password': '*********'}
>>> filter_extra(data['extra'])
>>> print(data)
{'extra': {'password': '123456'}}
```

If `filter_extra` returned the sanitized data, you could have something like `data['extra'] = filter_extra(data['extra'])`, but that's not the pattern followed by these filter methods.