`react-final-form` should be able to submit forms

So... bear with me for a minute. I had this absolutely crazy use case for a form library and I'm unsure how I got this way... so I'm sorry in advance.

I'd like to submit a form.

### What is the current behavior?
`react-final-form` does not submit forms

https://github.com/final-form/react-final-form/blob/a651c117cb1bfe448dfb1c11ebafa614ca285121/src/ReactFinalForm.js#L106-L108

The `handleSubmit` fn currently prevents form submission in 100% of cases. You can just.. not call `handleSubmit` and pass the `form` instance to your own handler function, running `form.submit()` - but if it finds a form error - and you don't `preventDefault()`, then the form still submits (so you lose the validation-before-submit behavior). If you `preventDefault` *after* calling `form.submit()` that "works", in the sense that your `react-final-form` `onSubmit` prop fn is invoked... but with no event or any way to complete the action. So I'm kind of at a loss. Only thing I can think of is manually running validation and then just letting the event complete / invoking `form.submit()`, but at that point you lose a lot of the libraries' utility.

### What is the expected behavior?
`react-final-form` should be able to submit a form.

The rest of the features work great - and I really like the design of the library. I just.. would like to be able to submit a form. I get that it's a bit more common use case to do some asynchronous thing or make a fetch request - but there are valid use cases for want to just make a post request. I assumed I was just missing something but I couldn't even find an escape hatch. The event itself seems to be kept pretty inaccessible. I'll take any advice (or submit a PR if you have a suggestion for how to implement the functionality)