Add checkbox component
Add additional form validation using custom function

This enables one to create a new schema based on given
data.

E.g. if you want to validate date2 is after given date1,
you can create a function which takes coerced map and returns
a schema where data2 is checked against s/pred which uses date1.
Add local state to field components

This enables defining state of component from the form render fn.
This can be used e.g. when date inputs config should depend on
external value, e.g. other date value.
Add min- and max-date params to datepicker.