Add support for white-listing the env, rather than just black-listing.

[mexisme]

We require support for white-listing env var's, as we've been caught-out in the past by things like credit-card details being unexpectedly leaked because the black-list wasn't adequately updated.

To support this, we created a fork off 1.0 (https://github.com/powershop/exception_notification/tree/ps1.0.1) that patched several things, but also included white-listing for email notifications.
We recently upgraded our code to v4.0.1 with the our patches (https://github.com/powershop/exception_notification/tree/ps4.0.1.1) though it's very email-specific, and mostly to our needs only.

We'd like to have this functionality rolled-into exception_notification in a more-abstract way, that can easily be used by multiple plugins.

I note there have been broadly similar issues/discussions, like #182 and #99

What would be the right way forward?