Feedback wanted - Add `at_exit` hook for auto exception capturing in scripts

Currently, automatic capturing only works in rake/rails applications.
We want to see if users also want out of the box exception capturing in one-time scripts and other ruby applications.

Raven had this [here](https://github.com/getsentry/sentry-ruby/blob/ee5eadc34187521259044ce548107a7e48711b0d/sentry-raven/lib/raven/instance.rb#L235-L243) and python has [`AtExitIntegration`](https://github.com/getsentry/sentry-python/blob/master/sentry_sdk/integrations/atexit.py).