Support the transient behavior for the linux backend

[rhabbachi]

Using ntfy with the Gnome desktop can very quickly fillup the panel's message tray.

This PR Adds support for making the notification "skip the history". The notification message will show up as usual, but it does not persist in the notification history if available (in my case the message tray).

This is achieved on Linux desktop by setting the "transient" hint: notify-send -t 1 --hint int:transient:1 "this is a test"

[dschep]

omg awsome! That's been bugging me too. I'll merge and tweak this a bit later and probably make transient the default for shell-integration and make the variable check based on truthyness (with special case of the string "false" being falsey)

[coveralls]

Coverage decreased (-0.8%) to 83.908% when pulling 885d717 on rhabbachi:linux-transient-opt into 7a79b95 on dschep:master.