Add some way to know if a toast is already visible to avoid multiple toasts

[BrendanBasson4]

Would it be possible to expose some kind of value for example Fluttertoast.isCurrentlyShowingToast that could be used to prevent multiple toasts from firing one after the other. Or perhaps a toast key parameter of some kind that will only show one toast if toasts have the same key?

[mayank29malhotra]

I have added this feature and added a pull request