feat: support to install toast module under namespaced module

[ktsn]

fix #11

This feature allows the user to install toast module under some namespaced module which the user defined.

Example:

import Vue from 'vue'
import Vuex from 'vuex'
import { createModule } from 'vuex-toast'

Vue.use(Vuex)

export default new Vuex.Store({
  modules: {
    libs: {
      namespaced: true,
      modules: {
        toast: createModule()
      }
    }
  }
})

In the example, the toast module is under libs module which is namespaced. So the all toast module assets (getters/actions/mutations) are prefixed by libs/.

In that case, we need to notify <Toast> component that the toast module has the prefix. To do that, we can use namespace prop of the component.

<Toast namespace="libs" />