Implement the horizontal rule parser.

src/parser/lesser_element.rs

@@ -79,6 +79,11 @@ pub struct FixedWidthArea<'s> {
     pub source: &'s str,
 }
 
+#[derive(Debug)]
+pub struct HorizontalRule<'s> {
+    pub source: &'s str,
+}
+
 impl<'s> Paragraph<'s> {
     pub fn of_text(input: &'s str) -> Self {
         let mut objects = Vec::with_capacity(1);
@@ -157,3 +162,9 @@ impl<'s> Source<'s> for FixedWidthArea<'s> {
         self.source
     }
 }
+
+impl<'s> Source<'s> for HorizontalRule<'s> {
+    fn get_source(&'s self) -> &'s str {
+        self.source
+    }
+}