fix(binding): Expose validator engine used by the default Validator (#1277)

* fix(binding): Expose validator engine used by the default Validator

- Add func ValidatorEngine for returning the underlying validator engine used
  in the default StructValidator implementation.
- Remove the function RegisterValidation from the StructValidator interface
  which made it immpossible to use a StructValidator implementation without the
  validator.v8 library.
- Update and rename test for registering validation
  Test{RegisterValidation => ValidatorEngine}.
- Update readme and example for registering custom validation.
- Add example for registering struct level validation.
- Add documentation for the following binding funcs/types:
  - Binding interface
  - StructValidator interface
  - Validator instance
  - Binding implementations
  - Default func

* fix(binding): Move validator engine getter inside interface

* docs: rm date cmd from custom validation demo

binding/default_validator.go

@@ -28,9 +28,13 @@ func (v *defaultValidator) ValidateStruct(obj interface{}) error {
 	return nil
 }
 
-func (v *defaultValidator) RegisterValidation(key string, fn validator.Func) error {
+// Engine returns the underlying validator engine which powers the default
+// Validator instance. This is useful if you want to register custom validations
+// or struct level validations. See validator GoDoc for more info -
+// https://godoc.org/gopkg.in/go-playground/validator.v8
+func (v *defaultValidator) Engine() interface{} {
 	v.lazyinit()
-	return v.validate.RegisterValidation(key, fn)
+	return v.validate
 }
 
 func (v *defaultValidator) lazyinit() {