This adds support for Gotify (https://gotify.net) notifications.
Work items:
* Two flags have been added to internal/flags/flags.go:
"notification-gotify-url" and "notification-gotify-token".
* A Gotify notification driver has been added in
notifications/gotify.go.
* "gotify" has been added to notification driver choices in
notifications/notifier.go.
* Docs have been updated
