Add ability to only rebuild things that actually need to be rebuilt.

[NHDaly]

Maybe integration with something like make?

Or maybe we can use julia's Pkg manager to do this?

Been thinking about hermetically including dependencies by recreating a .julia structure inside the .app (via JULIA_DEPOT_PATH), and if we do that, maybe we could wrap the user's code all up in a Package inside there, and only re-run things if it's modified somehow?