Add test coverage to core

core/user.go

@@ -0,0 +1,16 @@
+package core
+
+import "fmt"
+
+type User struct {
+	ID string
+}
+
+// Mention generated a Discord mention string for the user
+func (user *User) Mention() string {
+	if user == nil {
+		return "<NULL>"
+	}
+
+	return fmt.Sprintf("<@%s>", user.ID)
+}