feat(form): set optional field behavior

Adds the possibility to use $('.ui.form').form('set optional', fieldName, bool); to set or unset a field as optional for validation.

src/definitions/behaviors/form.js

@@ -1165,6 +1165,14 @@ $.fn.form = function(parameters) {
                 }
               }
             });
+          },
+          optional: function(identifier, bool) {
+            bool = (bool !== false);
+            $.each(validation, function(fieldName, field) {
+              if (identifier == fieldName || identifier == field.identifier) {
+                field.optional = bool;
+              }
+            });
           }
         },