fix: Match GetterHandler params to Vuex requirements

Copy of PR on vuex-typescript:
istrib/vuex-typescript#22
Fixes bug that did not allow users to access RootState from typed getters
Bug on vuex-typescript:
istrib/vuex-typescript#11

src/index.ts

@@ -6,11 +6,11 @@ export function Handler(target: any, key: string) {
     target[key]._vuexKey = key;
 }
 
-/**
- * Vuex getter handler specified in Vuex options.
+/*
+ * Vuex getter handler with all available parameters as specified in Vuex options
  */
 export type GetterHandler<TModuleState, TRootState, TResult> =
-    (state: TModuleState, rootState: TRootState) => TResult;
+    (state: TModuleState, getters: any, rootState: TRootState, rootGetters: any) => TResult;
 
 /**
  * Vuex action handler which takes payload as specified in Vuex options.