title: Submitting additional field attributes using Redux Form
body: <p>I have a redux-form driven form that's being constructed via JSON sent from my backend. This data has specific keys that I need to post back on submit (e.g. filterID). Redux-form appears to keep a fairly minimal state representation of the form/fields. As an example, here's a trivial state dump:</p>
  
  <pre><code>{form : {
      myForm: {
          registeredFields: {
              field1: {name: "field1", type: "Field", count: 1},
              field2: {name: "field2", type: "Field", count: 2}
          },
          values: {
              field1: "123"
          }
      }
  }
  </code></pre>
  
  <p>Is there a built-in way using redux-form that I could have it store additional data in the state (e.g. filterID), or key off of a attribute/prop than name? </p>
  
  <pre><code>&lt;Field
        key={somethingunique}
        name={component.name}
        component={component.type}
        filterID={component.id}
  /&gt;
  </code></pre>
  
  <p>This would yield something like the following state (move the additional stuff/filterID around anywhere as long as it's present):</p>
  
  <pre><code>{form : {
      myForm: {
          registeredFields: {
              field1: {name: "field1", type: "Field", count: 1},
              field2: {name: "field2", type: "Field", count: 2}
          },
          values: {
              field1: "123"
          },
          additionalStuff: {
              field1: {filterID: "abc"}
          },
      }
  }
  </code></pre>
  
  <p>My thinking with this approach is that it's easily grabbed in handleSubmit such that I can send it back to the server with a simple stringify.</p>
  
  <p>I can play some games with adding a big object to state and doing lookups on submission, connecting components, etc., but those feel messy.</p>