The `unload+` module allows modules to register callbacks that are called when they are unloaded, and associate unload functions to containers to have the unloader automatically deleted when the container unloads. @function Save callbacks to run when unloading in a unload queue. Optionally scope the callback to a container, e.g., window. Provide a way to run all the callbacks. @param callback {function} A function to be called when the module unloads. @param [container] {object} Optional container object; if the container "unloads" before the module unloads, then the associated callback is removed from the unload queue. @returns {function} Returns a function which will allow one to remove the callback from the unload queue.