Add `before_breadcrumb` hook

[limbonaut]

Allow customizing breadcrumbs through the before_breadcrumb hook. This is important for scrubbing sensitive data: https://docs.sentry.io/platforms/python/data-management/sensitive-data/

WIP PR: #83 -- put on hold, as it's not a priority.

Please 👍 if this is something you'd like to have support for.