Add an initial implementation of PlainLink.

src/parser/object_parser.rs

@@ -68,6 +68,7 @@ pub fn any_object_except_plain_text<'r, 's>(
             parser_with_context!(regular_link)(context),
             Object::RegularLink,
         ),
+        map(parser_with_context!(plain_link)(context), Object::PlainLink),
     ))(input)
 }