title: withFormik(): How to use handleChange
body: <p><strong>Current platform:  NodeJS (minimal), client-side React w/Redux, Formik, Yup.</strong></p>
  
  <p>Given the following example code (not including the entire React Component code since it's irrelevant to the question):</p>
  
  <pre><code>class RegisterPage extends React.Component {
      constructor(props) {
          super(props);
      }
  
      // (...)
  
      render () {
          &lt;Form&gt;
              &lt;Field name="email" type="email" /&gt;
              &lt;Field name="password" type="password" /&gt;
              &lt;Field 
                  name="myCheckbox" 
                  type="checkbox"
                  checked={this.props.values.myCheckbox}
                  onChange={  ??????????  } /&gt;
          &lt;/Form&gt;
      }
  }
  
  const handleFormSubmission = (values, { resetForm, setErrors, setSubmitting }) =&gt; {
      console.log(values);
  };
  
  const handleFormChange = (event) =&gt; {
      event.persist();
      console.log('changed');
  };
  
  const MyFormik = withFormik({
      mapPropsToValues ({ email, password, myCheckbox }) {
          return {
              email: email || '',
              password: password || '',
              myCheckbox: myCheckbox || false
         }
      },
      validationSchema: (...Yup schema here...),
      handleSubmit (values, bag) { return handleFormSubmission(values, bag); }
  })(RegisterPage);
  
  export default connect()(MyFormik);
  </code></pre>
  
  <p>...<strong>how can I use the handleChange method</strong>?  I need to persist the original one (the one from Formik) while adding code that handles that checkbox change.  There's some component behavior that depends on the checked value of that checkbox.</p>
  
  <p>Please notice that I'm not passing the onChange prop to email or password, since there is no extra behavior to code for on those.  The checkbox is the one that will have special behavior.</p>