Expose setting to handle prior firing of window.onload

In cases where a javascript module loader (such as requireJS) is being used to load jquery and jquery.sequence, the window.onload event may have fired prior to the jquery.sequence code being executed.  This causes the plugin to fail, as the plugin hangs indefinitely because it's own window.onload handler will never be triggered.

To counter this, I've exposed the windowLoaded variable in the plugin settings.  If the developer sets this to true, the developer is assuring jquery.sequence that the window.onload event has already fired.