* 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
* feat(binding): Add support for custom validation tags
* docs: Add example for custom validation tag
* test(binding): Add test for registering custom validation
ziheng