Fix Flow typings for UseFieldConfig and FieldProps (#503)

There was a mismatch between the source code and Flow typings which led to the following error:
```js
<Field
  name={name}
  component={SomeCustomInputComponent}
  /* ^^^ Flow error here:
  Cannot create Field element because in property component:
   • Either function [1] is incompatible with string literal
     input [2].
   • Or function [1] is incompatible with string literal
     select [3].
   • Or function [1] is incompatible with string literal
     textarea [4].
  */
```

Author: eugene-stativka

src/types.js.flow

@@ -75,7 +75,8 @@ export type UseFieldConfig = {
   afterSubmit?: () => void,
   allowNull?: boolean,
   beforeSubmit?: () => void | false,
-  component?: SupportedInputs,
+  children?: $PropertyType<RenderableProps<*>, 'children'>,
+  component?: $PropertyType<RenderableProps<*>, 'component'>,
   defaultValue?: any,
   format?: (value: any, name: string) => any,
   formatOnBlur?: boolean,
@@ -92,9 +93,7 @@ export type UseFieldConfig = {
 
 export type FieldProps = UseFieldConfig & {
   name: string
-} & RenderableProps<FieldRenderProps> & {
-    component?: React.ComponentType<*> | SupportedInputs
-  }
+} & RenderableProps<FieldRenderProps>
 
 export type UseFormStateParams = {
   onChange?: (formState: FormState) => void,