feat(binding): add support for custom validator / validation tags (#1068)

* feat(binding): Add support for custom validation tags

* docs: Add example for custom validation tag

* test(binding): Add test for registering custom validation

binding/binding.go

@@ -4,7 +4,11 @@
 
 package binding
 
-import "net/http"
+import (
+	"net/http"
+
+	validator "gopkg.in/go-playground/validator.v8"
+)
 
 const (
 	MIMEJSON              = "application/json"
@@ -31,6 +35,11 @@ type StructValidator interface {
 	// If the struct is not valid or the validation itself fails, a descriptive error should be returned.
 	// Otherwise nil must be returned.
 	ValidateStruct(interface{}) error
+
+	// RegisterValidation adds a validation Func to a Validate's map of validators denoted by the key
+	// NOTE: if the key already exists, the previous validation function will be replaced.
+	// NOTE: this method is not thread-safe it is intended that these all be registered prior to any validation
+	RegisterValidation(string, validator.Func) error
 }
 
 var Validator StructValidator = &defaultValidator{}