feat: allow to passing functions in mapActions/mapMutations (fix #750)

[ktsn]

This is a small improvement for mapActions and mapMutations helpers. It allows users to pass functions in the helpers so that reducing some redundant method definitions. The feature is like a counterpart of passing function in mapState.

For example:

const vm = new Vue({
  store,
  template: `<input @input="onInput">`
  methods: mapActions({
    onInput (dispatch, event) {
      // First argument is as same as this.$store.dispatch
      dispatch('inputText', {
        text: event.value
      })
    }
  })
})

The above code is equivalent with the below (Note that inputText is a bit redundant since it is just an bridge method between onInput and the action):

const vm = new Vue({
  store,
  template: `<input @input="onInput">`
  methods: {
    ...mapActions(['inputText']),
    onInput (event) {
      this.inputText({
        text: event.value
      })
    }
  })
})

In addition, this syntax is more effective when we map namespaced modules.

methods: mapActions('some/module', {
  onInput (dispatch, event) {
    // `dispatch` is already namespaced here.
    // So the below line will call `some/module/inputText` action.
    dispatch('inputText', {
      text: event.value
    })
  }
})

I did not implement this syntax for mapGetters because we already can do the same thing with mapState.

Close #750

[rayax86]

Hi @ktsn, could you please elaborate on

I did not implement this syntax for mapGetters because we already can do the same thing with mapState.

?

What do you mean by "we already can do the same thing with mapState"?

A little bit of context: I am in a situation where I wish to dynamically map getters to one computed property, depending on a piece of data. So it would be nice to:

...mapGetter({
    someValue: function(getters) {
        return getters[this.getterName]
    }
}

[rayax86]

Nevermind, I now see getters are passed to mapState as a param.

Would be nice if the documentation mentions this though.