feat: add c.BindCookie(obj) from cookie, and c.BindRequest(obj) from whole request

[tangx]

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in order to bind values into object more simple.

1. adding c.BindCookie(obj) to get parameters from cookies.

2.1. extract original Binding code into BindOnly and BindBodyOnly method, these two methods are only binding parameters into obj.
2.2. Bind method handler error from BindOnly method, then validate the obj.
2.3 BindOnly and BindBodyOnly are public methods, so that users can define their own logic without validation (binding:"required") checking trouble.

type Binding interface {
	Name() string
	Bind(*http.Request, interface{}) error
	BindOnly(*http.Request, interface{}) error
}

// BindingBody adds BindBody method to Binding. BindBody is similar with Bind,
// but it reads the body from supplied bytes instead of req.Body.
type BindingBody interface {
	Binding
	BindBody([]byte, interface{}) error
	BindBodyOnly([]byte, interface{}) error
}

3. add a new method c.BindRequest to bind all parameters in request into obj at once. By using original Binding and BindingBody method now supported.
   now supports tag are form, cookie, header, body.

4. add and pass testing functions.

5. add documents about these 2 methods.