Add initialXXXX and fix isValid

[jaredpalmer]

• Deprecate isInitialValid with a warning. If it is specified, old 1.x isValid behavior is utilized.
• isValid is now true by default.
• Add initialErrors and initialTouched props to <Formik>
• Add mapPropsToErrors and mapPropsToTouched to withFormik()
• Breaking: resetForm(nextValues: Values) to resetForm(nextInitialState: FormikState<Values>

I'm not convinced we actually need to change resetForm (and thus keep all of these as refs that can be mutated on reset)

FWIW I can't think of when you would want initialTouched. Open to ideas and would be happy to remove.

Closes #1133 #1116 #1148 #288 #954 #698

[jaredpalmer]

Does anyone actually need initialTouched?

[ohmonster]

I think I need initial touched. We have a use case where users may have address data saved on the server that doesn't meet our current validation field criteria --generally missing a phone number or something. Our error display rule triggers if the field is touched. So we want the form to be as populated as it can be, validated on mount, and touched on mount. -- We reuse the form, so we want this to happen conditionally as well. Ugh.

I was using a ref to do this before, but the upgrade to Formik 2.0 gave me errors about innerRefs.