Moved rules into their own files

Everything under `lib/rules/*` is a rules file (with the name of the rule in camelCase), re-exported into an array in `lib/rules.js`.
Moved the regular expressions from `lib/rules.js` to `lib/expressions.js`, and the rest of the variables into `lib/shared.js`.

lib/rules/noDuplicateHeader.js

@@ -0,0 +1,19 @@
+var shared = require("../shared");
+
+module.exports = {
+  "name": "MD024",
+  "desc": "Multiple headers with the same content",
+  "tags": [ "headers" ],
+  "aliases": [ "no-duplicate-header" ],
+  "regexp": null,
+  "func": function MD024(params, errors) {
+    var knownContent = [];
+    shared.forEachHeading(params, function forHeading(heading, content) {
+      if (knownContent.indexOf(content) === -1) {
+        knownContent.push(content);
+      } else {
+        errors.addContext(heading.lineNumber, heading.line.trim());
+      }
+    });
+  }
+};