Use FieldConfig.isEqual to determine if radio button is checked

### Are you submitting a **bug report** or a **feature request**?

A wee little feature request

### What is the current behavior?

The documentation says: "The radio button will render as checked if and only if the value given here === the value for the field in the form."

Is it reasonable to have FieldConfig.isEqual used instead of "===", when present? I'm storing an object as the radio input value and would like to set the equality comparison to (a, b) => { return (a.id === b.id); }

### What's your environment?

"final-form": "^4.10.0",
"react-final-form": "^3.6.5"

### Other information

<3 react-final-form!