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/json.go

@@ -10,6 +10,9 @@ import (
 	"github.com/gin-gonic/gin/json"
 )
 
+// EnableDecoderUseNumber is used to call the UseNumber method on the JSON
+// Decoder instance. UseNumber causes the Decoder to unmarshal a number into an
+// interface{} as a Number instead of as a float64.
 var EnableDecoderUseNumber = false
 
 type jsonBinding struct{}