Make sure that multiple instances of ok aren't running

This commit makes sure that multiple instances of ok aren't running. Here's how it works:

- It checks for a file called .ok_lock. If it's there, then it asks the user to check if another ok is running somewhere. If it isn't, then they should type in 'Run ok anyway'. If not, they can type in anything else and it'll exit.

- If no file called .ok_lock exists, it makes it and removes it at the end of the ok run.
The term "tab" is probably too specific -- this problem would occur even if students have multiple terminal windows open. Something like "multiple instances of ok running at the same time."