Write a function that accepts a function and timeout, x, in number of milliseconds. It returns a function that can only be executed once per x milliseconds. This can be useful for limiting the number of time and computation heavy function that are run. For example, making AJAX requests to an autocompletion API.

Once written, add a third parameter that will allow the function to be executed immediately if set to true. Otherwise the function will run at the end of the timeout period.