Move each rule implementation into its own file (fixes #83).

lib/md024.js

@@ -0,0 +1,22 @@
+// @ts-check
+
+"use strict";
+
+var shared = require("./shared");
+
+module.exports = {
+  "names": [ "MD024", "no-duplicate-header" ],
+  "description": "Multiple headers with the same content",
+  "tags": [ "headers" ],
+  "function": function MD024(params, onError) {
+    var knownContent = [];
+    shared.forEachHeading(params, function forHeading(heading, content) {
+      if (knownContent.indexOf(content) === -1) {
+        knownContent.push(content);
+      } else {
+        shared.addErrorContext(onError, heading.lineNumber,
+          heading.line.trim());
+      }
+    });
+  }
+};