title: vuex subscribe to individual mutation
body: <p>Is it possible to subscribe to an individual mutation?</p>
  
  <p>Instead of:</p>
  
  <pre><code>this.$store.subscribe((mutation, state) =&gt; {
     if(mutation === 'someMutation'){
         doSomething()
     }
  })
  </code></pre>
  
  <p>I would like something like this:</p>
  
  <pre><code>this.$store.subscribe('someMutation', state =&gt; {
      doSomething()
  })
  </code></pre>