Add helper function for setting cornerRadius

[cozzin]

Fix #113
Just add a setter for cornerRadius.
Actually, I have tried to make public backgroundView, but it does not work.

[devxoul]

I think this can be a good alternative solution.

[Coeur]

I'm not a fan of this. In case of ambiguity, there is certainly a way to specify which method is the desired one. And solving the ambiguity for calling our method will not solve the ambiguity when you want to call the UIView method. It's better to remove cornerRadius entirely as I wrote in #113

[cozzin]

@Coeur I know you want to make backgroundView as pulbic, but it does not work when I tested.

I tested like this:

ToastView.appearacne().backgroundView.layer.cornerRadius = 20

Or did you mean like this code?

ToastView.appearacne().backgroundView = CustomBackgroundView()