[structs] change package structutil to structs (#81)

* refactor package structutil to structs

* add structs package zh-CN docs

structs/tag.go

@@ -0,0 +1,36 @@
+package structs
+
+import (
+	"strings"
+
+	"github.com/duke-git/lancet/v2/validator"
+)
+
+// Tag is abstract struct field tag
+type Tag struct {
+	Name    string
+	Options []string
+}
+
+func newTag(tag string) *Tag {
+	res := strings.Split(tag, ",")
+	return &Tag{
+		Name:    res[0],
+		Options: res[1:],
+	}
+}
+
+// HasOption check if a struct field tag has option setting.
+func (t *Tag) HasOption(opt string) bool {
+	for _, o := range t.Options {
+		if o == opt {
+			return true
+		}
+	}
+	return false
+}
+
+// IsEmpty check if a struct field has tag setting.
+func (t *Tag) IsEmpty() bool {
+	return validator.IsEmptyString(t.Name)
+}