Allow setting exclude_fields through the bundle configuration

[doenietzomoeilijk]

The SlackRecord used by the Slack and SlackWebhook handlers has a constructor parameter to exclude certain fields from being sent to Slack. This is a great way to log exceptions but not have them clutter up your Slack channels, for example. Currently, the configuration for the bundle doesn't have a key for this field, which means it can't be configured.

There's been a PR open (#402 by @Gwemox), but for some reason it's just been sitting there, gathering dust for some two years. It'd be great if someone could either merge it, or, in case that's not an option, offer some guidance on What To Do to get it in a mergable state.