Feature/map fields accept function and map entire objects

[guilemos]

1. Support to map only one row that lives at a array/object when we are out of a loop and map all object properties

This is useful when working with "active row".

const state = {
  byId: {
    "/rooms/1": {
        name: "Nice room"
    },
    "/rooms/2": {
        name: "Nice room 2"
    },
  },
  allIds: ["/rooms/1", "/rooms/2"],
};

...mapFields('rooms', { item: 'byId[/rooms/2]' }),

<template>
    <v-input v-model="item.name" />
</template>

2. Support to accept functions as path for mapFields and mapMultiRowFields
   This is useful to make map dynamic.

const state = {
  byId: {
    "/rooms/1": {
        name: "Nice room"
    },
    "/rooms/2": {
        name: "Nice room 2"
    },
  },
  allIds: ["/rooms/1", "/rooms/2"],
};

data() {
  return {
    ative_room: '0'
  };
},
computed: {
  ...mapFields({name: (c) => `users[${c.ative_room}].name` }),
}

<div>
   <input v-model="name">
</div>

Thank you

[coveralls]

Coverage remained the same at 100.0% when pulling 50e3c03 on guilemos:feature/mapFields-accept-function-and-map-entire-objects into ae30a03 on maoberlehner:master.