Loosen typing requirements of init params, to allow tuples, sets, etc. of strings

[bskinn]

Just needs to be an iterable collection of strings, I think. Shouldn't specifically need the ordering or mutability of the list. Can't really picture a case where one would pass a dict, but who knows..?

Could then change the validator just to issubclass(type(arg), collections.Iterable), instead of an explicit type(arg) == list check.