[feature request] Allow manipulation of data between the form and submission

[keithradford]

You can currently pass a data object into useForm which allows you to merge additional data from outside the form into the submission.

It would be nice if, similarly to this, we could pass callback functions which perform operations on data from our form before submission.

For example, if I have a dropdown for selecting a state:

• user selects Washington
• user submits form
• callback converts Washington into WA
• WA is sent to Formspree

This could be a new attribute of the options PR, or potentially could be inferred from the existing data attribute.