First implementation of macro.

build.rs

@@ -58,7 +58,7 @@ fn write_header(test_file: &mut File) {
     write!(
         test_file,
         r#"
-#[feature(exit_status_error)]
+#![feature(exit_status_error)]
 use organic::compare_document;
 use organic::parser::document;
 use organic::emacs_parse_org_document;