style: add component 'rule' for horizontal file delimiter

tests/snapshots/output/changes_rule.snapshot.txt

@@ -0,0 +1,22 @@
+  struct Rectangle {
+      width: u32,
+      height: u32,
+  }
+  
+_ fn main() {
+      let rect1 = Rectangle { width: 30, height: 50 };
+  
+      println!(
+~         "The perimeter of the rectangle is {} pixels.",
+~         perimeter(&rect1)
+      );
++     println!(r#"This line contains invalid utf8:  "<22><><EFBFBD><EFBFBD><EFBFBD>"#;
+  }
+  
+  fn area(rectangle: &Rectangle) -> u32 {
+      rectangle.width * rectangle.height
+  }
++ 
++ fn perimeter(rectangle: &Rectangle) -> u32 {
++     (rectangle.width + rectangle.height) * 2
++ }