[Validation] Add ability to add a rule to a field after initializing validation

[jemarlin]

We had need in our project to conditionally add form validation based on workflow steps. We added this method to form module.add:

validation: function ( $field, rule ) {

  var
    identifier = $field.attr( 'id' ) || $field.attr( 'name' ) || $field.attr( 'data-' + metadata.validate ),
    validator
  ;

  if ( !validation || !$field.length) {
      return false;
  }

  validator = validation[identifier];
  if ( !validator ) { // no rules exist for field, create an empty rule set
      validator = validation[identifier] = { 'identifier': identifier, 'rules': [] };
  }

  validator.rules.push( rule ); // add the rule

  return true;

  },

Which can be called like so $form.form( 'add validation', $field, { 'type': 'empty', 'prompt': 'This field must contain a value' } );

[jlukic]

I've added add field add rule add fields remove rule remove fields in next version, check release notes and above commits for details.

I've also added an example in next doc update.