[feature] support single-timer-mode

[masukomi]

I find i generally don't want multiple timers to be able to run at the same time, but I can see value in the feature, so I don't want to get rid of it.

Instead i think i want to add a new metadata field to toggle "single timer mode"

in single timer mode the following behavior would be exhibited

starting 1 timer would stop a running timer at the same moment (or maybe prior second), and print out the ids of both. Something like:

stopped: @foo (123) after 22 minutes
started: @bar at 11:22