Implement {{ date }} variable for entries

src/entry.rs

 
 impl Entry {
     fn render(&self, template: &str, config: &Config) -> String {
+        let formatted_date = match self.date {
+            Some(date) => date.format("%Y-%m-%d").to_string(),
+            None => String::from(""),
+        };
         template
             .replace(
                 "{{ page_title }}",
                 "{{ body }}",
                 &markdown_to_html(&self.body, &ComrakOptions::default()),
             )
+            .replace("{{ date }}", &formatted_date)
     }
 }
 
         assert_eq!(post.body, "This is the body");
         assert_eq!(post.date, None);
     }
+
+    #[test]
+    fn test_display_entry_date() {
+        let post = Entry {
+            kind: EntryKind::Post,
+            title: String::from("Test Title"),
+            slug: String::from("one"),
+            body: String::from("This is the body"),
+            date: Some(NaiveDate::from_ymd(2000, 1, 1)),
+        };
+
+        let config = Config {
+            site_name: String::from("Test Site"),
+        };
+
+        assert_eq!("2000-01-01", post.render("{{ date }}", &config));
+    }
 }