Use a single function for finding all keywords.

src/types/lesser_element.rs

@@ -87,6 +87,8 @@ pub struct HorizontalRule<'s> {
 #[derive(Debug)]
 pub struct Keyword<'s> {
     pub source: &'s str,
+    pub key: &'s str,
+    pub value: &'s str,
 }
 
 #[derive(Debug)]