As requested, I've made the API easily extendable to handle alternative methods if those are deemed necessary.  Also, it is easy to change the default.

That said, it is my hope that these two methods and the current default get kept.  They have several virtues:  easy to explain, obviousness, widely adopted (most of the  other packages offer at least of these two methods), and they have most of the desired properties listed in Hyndman & Fan.  They are a good practical choice.