FormMixin: document validation prop
validation: remove isSuccess/isFailure attributes from validation

Instead one should use `validation.isSuccess(validation)` and
`validation.isFailure(validation)`. That makes validation values easier to
construct, we need it to pass external validation data to form.
Update value only in case it is valid
standalone: fix shim for global React
validation: relext isFailure
Add external validation (#7)

  - Add `externalValidation` prop for `Form` component
  - Provide `externalValidation` via `externalValidation()` via context
  - Field component now render external validation failure
tests: Make lint happy
standalone: Makefile gardening
lens: add isUndefined() method

This method checks if the current value is undefined. This workaround the
default value which can be returned by .val().
Field: render validation errors only if value is defined
standalone: fix Makefile to publish to different branches
docs: update family editor with required fields
docs: family editor now has custom error messages