title: Combining React Final Form Array and Final Form Calculate
body: <p>I am having a really hard time accessing the objects within the field array for a calculated field price * quantity = total . Have tried a bunch of regex for the decorated field including <code>field: /invoiceItems\[\d+\]/ ,</code> to access and the solution from this PR (<a href="https://github.com/final-form/final-form-calculate/pull/21/commits/dab00f8125079fed402712a4b0ed71663be0ba14" rel="nofollow noreferrer">https://github.com/final-form/final-form-calculate/pull/21/commits/dab00f8125079fed402712a4b0ed71663be0ba14</a>) </p>
  
  <p>In the PR I cant access the index. What is the best way to do a calculation with final form calculate and arrays? I have tried this in Formik as well but ran into the same issue. </p>
  
  <p>Code</p>
  
  <p>Form: </p>
  
  <pre><code>          onSubmit={onSubmit}
            decorators={[calculator]}
            mutators={{
              ...arrayMutators,
            }}
            initialValues={{
              companyName: "",
              companyAddress: "",
              companyCity: "",
              companyState: "",
              companyZip: "",
              invoiceNumber: shortid.generate(),
              invoicePaymentStatus: "",
              invoiceStatus: "",
              invoiceItems: [
  
              ],
              invoiceDate: new Date(),
              invoiceDueDate: new Date(
                new Date().getTime() + 7 * 24 * 60 * 60 * 1000
              ),
              clientFname: "",
              clientLname: "",
              billingAddress: "",
              billingCity: "",
              billingState: "",
              billingZip: "",
              propertyAddress: "",
              propertyCity: "",
              propertyState: "",
              propertyZip: "",
              invoiceTotal: "",
              tax: "",
            }}
            render={({
              handleSubmit,
              reset,
              submitting,
              pristine,
              values,
              form: {
                mutators: { push, pop },
              },
            }) =&gt; (
              &lt;form onSubmit={handleSubmit} noValidate&gt;
  </code></pre>
  
  <p>Field Array: </p>
  
  <pre><code>                            type="button"
                              onClick={() =&gt; push("invoiceItems", undefined)}
                            &gt;
                              Add Line Item
                            &lt;/button&gt;
                            &lt;FieldArray name="invoiceItems"&gt;
                              {({ fields }) =&gt;
                                fields.map((item, index) =&gt; (
                                  &lt;div key={item}&gt;
                                    &lt;Field
                                      name={`${item}.description`}
                                      component={TextField}
                                      placeholder="Description"
                                    /&gt;
                                    &lt;Field
                                      name={`${item}.price`}
                                      component={TextField}
                                      placeholder="Price"
                                    /&gt;
                                    &lt;Field
                                      name={`${item}.quantity`}
                                      component={TextField}
                                      placeholder="Quantity"
                                    /&gt;
                                    &lt;Field
                                      name={`${item}.total`}
                                      component={TextField}
                                      placeholder="Total"
                                    /&gt;
                                    &lt;span
                                      onClick={() =&gt; fields.remove(index)}
                                      style={{ cursor: "pointer" }}
                                    &gt;
                                      ❌
                                    &lt;/span&gt;
                                  &lt;/div&gt;
                                ))
                              }
                            &lt;/FieldArray&gt; 
  </code></pre>
  
  <p>Here is some of the Decorator I have tried. (Its a disaster with so many different attempts &amp; ignore the math as we were just trying to get it to work)</p>
  
  <pre><code>
      { 
        field: /invoiceItems\[\d+\]/ ,
        updates: {(name, index, value) =&gt; {
          [`invoiceItems[${index}].total`]: (_ignored, values) =&gt;  2 
          //invoiceItems.total: (price, allValues) =&gt; price * 2
        }
      }
    )
  </code></pre>