title: Adding custom prop to RadioGroup field with React Final Form
body: <p>I am making a <code>RadioGroup</code> and as I understand it the value has to be a string but I want a number so I can do some math with other component inputs.</p>
  
  <p>Would rather just have a custom prob like <code>carPrice</code> below but failing that some other way is fine. Any ideas please? this is driving me crazy now. Not found anything in the docs for ReactJS or Final Form about doing it or anywhere else I have looked.</p>
  
  <p>The commented lines are just some things I have tried as examples but there has been many more.</p>
  
  <p>I know how to do this with jQuery but I really wanted to get into React more. Especially Final Form.</p>
  
  <p>Of course, the most obvious thing would be to use the value with a number integer/float but that throws the error related to the protected 'type' having to be a string.</p>
  
  <p>Do I need to go as far as making a handler and what kind? Surely there is a way to just add it as a custom prop to the field?</p>
  
  <p>I could do this with jQuery but I really wanted to stick to ReactJS and Final form.</p>
  
  <p>Been trying to work this out for so long I am going code blind to solutions.</p>
  
  <pre class="lang-html prettyprint-override"><code>&lt;RadioGroup row&gt;
    &lt;FormControlLabel
      label="Ford"
      control={
        &lt;Field
          name="Ford"
          component={Radio}
          type="radio"
          defaultValue={0}
          value={11}
          //onClick={e =&gt; alert(e.target.value)}
          //carPrice={555}
          // value={`${55}`}
          // value={888 ? String(yourValue) : null}
          // props = {{ myPrice: 475 }}
  
        /&gt;
      }
    /&gt;
  </code></pre>