type Getter<S,R> should declare optional parameters

[kbirger]

Currently, the definition is:

export type Getter<S, R> = (state: S, getters: any, rootState: R, rootGetters: any) => any;

every parameter other than state: S is actually optional, and should be declared as such.

Right now this is keeping me from actually using these types, because using them would make my unit test code messy with extraneous null-valued params.

Also, could you provide some guidance on when these typings are actually useful? As far as I know, these classes are never used directly, but rather by Vuex internals.