fix metadata parsing, accounting for trailing quotes

src/markdown.hpp

@@ -13,6 +13,8 @@ struct Metadata {
 // Metadata are in yaml format at the top of the file
 // They are separated from the rest of the file with `---` at the beginning and
 // at the end.
+//
+// Returns nullopt when no metadata is found in the file
 std::optional<Metadata> get_metadata(const std::string &FILE_PATH);
 
 // Reads the markdown file and converts it to html.