on_idle reads a list of time-outs (in seconds) paired with commands
from standard input, then executes them when the mouse cursor becomes
idle.
Lines should look like this:
20 echo Stop slacking off, you lazy bum!
40 echo If you don't move the mouse cursor right now ...
41 echo I'm going to delete your ssh keys from memory!
50 echo Last chance!
60 ssh-add -D