context:add Bind function

// Bind data from request.Form[key] to dest
// like
/?id=123&isok=true&ft=1.2&ol[0]=1&ol[1]=2&ul[]=str&ul[]=array&user.Name=
astaxie
// var id int  beegoInput.Bind(&id, "id")  id ==123
// var isok bool  beegoInput.Bind(&isok, "isok")  id ==true
// var ft float64  beegoInput.Bind(&ft, "ft")  ft ==1.2
// ol := make([]int, 0, 2)  beegoInput.Bind(&ol, "ol")  ol ==[1 2]
// ul := make([]string, 0, 2)  beegoInput.Bind(&ul, "ul")  ul ==[str
array]
// user struct{Name}  beegoInput.Bind(&user, "user")  user ==
{Name:"astaxie"}