copy_notification_images.sh: more changes
copy_notification_images.sh: more changes

* Created usage_and_exit() function to centralize displaying the usage.
* Added "--help" option.
* Renamed "NOTIFICATION_FILETYPE" to "NOTIFICATION_TYPE" since it's not a file type.
* Changed "expires_in_seconds" from being after the notification type on the command line, to being "--expires seconds".  This allowed the next change...
* Added ability to create custom notifications when "notification_type" is "custom".  The "custom_args" command-line arguments specify the attributes of the notification.  allsky.sh and the capture* programs will use that feature in a separate PR.
thomasjacquin/allsky,thomasjacquin/allsky,thomasjacquin/allsky