FR: Support DBUS URL's

[SpiderUnderUrBed]

It would be useful if you could use shoutrrr for local notifications, something like:
shoutrrr send -u "dbus://" -m "Hello from Shoutrrr!"
There is no need for platform specific notifications like kde or gnome as notify-send or dbus commands can abtract away that stuff. So supporting a DBUS url should tell shoutrrr to send notifications like this:

gdbus call --session \
    --dest org.freedesktop.Notifications \
    --object-path /org/freedesktop/Notifications \
    --method org.freedesktop.Notifications.Notify \
    "AppName" 0 "" "Hello from DBus" "This is a KDE notification" \
    [] {} 5000