Shell integration: use different backends for different length commands

[jayrhynas]

One useful feature would be to change the backend used depending on how long a command takes. For example, if a command takes <5 min, I only want to show local notifications, but if it's >5 min I want to also send a remote notification with pushover.

[dschep]

I like the idea, we'll see if I get around to it :wink: