Add existingState property to Form component

[williamcotton]

Adds existingState property to the <Form /> component.

Pre-populates a form with existing data in a manner consistent with form.getValues().

import Form from 'react-validation/build/form';

const state = {
  title: "Existing Title",
  description: "An existing description.",
};

<Form existingState={state} />
  <TextInput name="title" validations={[required]} />
  <TextArea name="description" validations{[required]} />
</Form>

This is useful for CRUD applications where the same form will be used for both creating and editing.