set form state from on props update

[jschr]

Currently createForm is only called on componentWillMount. If props change and the form's initial state depends on props it won't be updated.

Add a value prop to <FormProvider> to allow for updating the form state if props have changed. Also paves the way for improved composability.

<FormProvider form={form} value={props.user} onSubmit={saveUser}>
...
</FormProvider>

[jschr]

This may make withForm accepting a function of props redundant.