reaper: add support for a pre-delete hook

[yanniszark]

Hi @yosssi! Again, thanks a lot for this package, it has been very useful to us for this project:
https://github.com/arrikto/oidc-authservice

A new need has come up to do additional work before a session is reaped.
For example, we need to revoke tokens at a remote endpoint.
I have prepared and will open a PR for this functionality, if you are willing to accept.
I understand that you may not have the time to help, in which case we would have to use our own fork.