Add filters

[zachgoldstein]

In airbrake-ruby, we have https://github.com/airbrake/airbrake-ruby#airbrakeadd_filter. It would be nice to provide a flexible way for users to similarly filter out sensitive information.

[zachgoldstein]

There's a couple different types of filters to build here:

• White/black list keys, and replace their contents with [Filtered].
• https://github.com/airbrake/airbrake-ruby#airbrakeadd_filter where we add a user-supplied callback that does more complex things.
• Class-based filters, where you pass in an instance of the filter.

Each one of these is probably a PR, and I think the order above makes sense from simple to more complicated.