title: Svelte: transition on reactive data change
body: <p>What would be the best way to trigger an animation when a reactive variable changes?
  I would like to do something like this : </p>
  <pre class="lang-html prettyprint-override"><code>&lt;script&gt;
      import { fade } from 'svelte/transition'
      let count = 0;
      const handleClick = () =&gt; count +=1
  &lt;/script&gt;
  
  &lt;button on:click={handleClick} transition:slide&gt;
      Click me
  &lt;/button&gt;
  &lt;p&gt; You cliked &lt;strong transition:fade&gt;{count}&lt;/strong&gt; times&lt;/p&gt;
  </code></pre>
  <p>Which doesn't work because the <code>&lt;strong&gt;</code>node isn't removed from the DOM (I guess). So what would be the best way to have numbers fading in and out when they change?</p>