title: Modify props.value from within child component
body: <p>I am new to Vue and trying to build a "dropdown" component. I want to use it from a parent component like this:</p>
  
  <pre><code>&lt;my-dropdown v-model="selection"&gt;&lt;/my-dropdown&gt;
  </code></pre>
  
  <p>where <code>selection</code> is stored as <code>data</code> on the parent, and should be updated to reflect the user's selection. To do this I believe my dropdown component needs a <code>value</code> prop and it needs to emit <code>input</code> events when the selection changes.</p>
  
  <p>However, I also want to modify the <code>value</code> from within the child itself, because I want to be able to use the dropdown component on its own (I need to modify it because otherwise the UI will not update to reflect the newly selected value if the component is used on its own).</p>
  
  <p>Is there a way I can bind with <code>v-model</code> as above, but also modify the value from within the child (it seems I can't, because <code>value</code> is a prop and the child can't modify its own props).</p>