Ability to use a custom hook

[TangentFoxy]

My use case right now involves using the sandbox not to stop infinite loops, but to make sure an outside condition can stop whatever's running in the sandbox (I'm using the Love2D engine, which allows me to run threads, so my stop condition is based on another thread running concurrently).

I will implement this myself within my project, but I will also send a pull request if I feel it is worth sharing, and if anyone else finds it useful.