Add methods to update form data manually

[gwhobbs]

Sometimes it's helpful to be able to update form data manually. For example, maybe:

• We might want changing one field to affect the values of other fields (e.g. an array of values that must add to 100)
• We might want a button that causes predefined changes to happen to the form data (e.g. a button that alters existing items in an array)

This PR experimentally adds two new features addressing these issues:

1. A custom onApplyChangeToForm(formData: FormData, fieldValue: any, fieldPath: string) => FormData updater function can be provided as an option in getInputProps()
2. An updateFormData(formData: FormData) => void function is exposed for making arbitrary changes to the form data.

Closes #41