add Tag method to user (#364)

* add Tag method to user

* change format verb to %s

* use consistent string forming

user.go

@@ -1,5 +1,7 @@
 package discordgo
 
+import "fmt"
+
 // A User stores all data for an individual Discord user.
 type User struct {
 	ID            string `json:"id"`
@@ -15,5 +17,10 @@ type User struct {
 
 //String returns a unique identifier of the form username#discriminator
 func (u *User) String() string {
-	return u.Username + "#" + u.Discriminator
+	return fmt.Sprintf("%s#%s", u.Username, u.Discriminator)
+}
+
+//Mention return a string which mentions the user
+func (u *User) Mention() string {
+	return fmt.Sprintf("<@%s>", u.ID)
 }