add some more apis like vuex-orm  #480

Hi,

I found that vuex-orm is heavily inspired by redux and it has also huge list of api like firstOrCreate, upsert, hasmanythrough, polymorphic relationship etc,

Also Vuex-orm let user to access model from anywhere and call model methods but in redux-orm we have to call within reducer only, can't call something like Book.create() from saga, thunk or may be from component,

can it be done ? if yes, it would be too much helpful and powerful