[Experimental] Handle interruptions

[Vivelin]

Pink inspired me by sometimes tracking multiple items in rapid succession — issues commands while Tracker is still responding to the previous command. I figured it'd be fun to add a little flair by having Tracker interrupt these kinds of situations with some sass.

Considerations:

• How many times should Tracker tolerate being interrupted before speaking up about it?
• Should we add a timeout before speaking up again, in addition to the threshold?
  • It's not likely to happen too often. And if it does happen often, we should increase the threshold instead. But still, it's worth considering.
• I've added Feature toggles #97 so for similar ideas in the future, we can experiment more easily and give users the ability to toggle something off.