Add support for notification overrides

[jeffbargmann]

Host may wish to silence notifications, or hide the body of notifications for privacy.

[seriema]

Looks very interesting! Could you help out with some documentation on the Readme.md? If you have some example it would be great to include it in https://github.com/seriema/electron-notification-shim-demos :D

[seriema]

@JBLatenight ping

[seriema]

@JBLatenight I'm using your code as an inspiraton for a new feature. It's on the branch notification-onclick-override. I still have questions about your code so I might not take it as-is.

[khaled-su]

@seriema Any progress here? Is the branch ready? Thanks.

[seriema]

@khaledabbas Sadly, no. You can read my thoughts on it on the feature request #4 as to why I'm a bit stuck atm. Any help is welcome.