Fixes #82 - Add timeout option

[uzairfarooq]

This PR adds a new timeout option to both arrive and leave events that automatically unbinds the event and calls the callback with null after the specified number of milliseconds if no matching elements are found/removed.

This is useful for cases where you want to stop watching for elements after a certain time period.

Usage

// Will call callback with null after 2 seconds if no matching element is found
document.arrive(".dynamic-content", { timeout: 2000 }, function(elem) {
    if (elem === null) {
        console.log("Element not found within timeout");
        return;
    }
    console.log("Element found:", elem);
});

Related Issues

Closes #82