Pre-release v4.0.0-rc3

• Use *validation.Context in validation message placeholders. The validation.Placeholder signatures becomes func(fieldName string, language string, ctx *validation.Context) string. This is a breaking change.
• Added Extra (map[string]interface{}) to *validation.Context. This allows placeholders to use additional information given by the validator function.
• Added validation.StructList. This acts as a better alternative syntax for validation. The previous alternative syntax (*validation.Rules) should not be used for validation rules definition anymore but still works.