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/singleH1.js

@@ -0,0 +1,23 @@
+var shared = require("../shared");
+
+module.exports = {
+  "name": "MD025",
+  "desc": "Multiple top level headers in the same document",
+  "tags": [ "headers" ],
+  "aliases": [ "single-h1" ],
+  "regexp": null,
+  "func": function MD025(params, errors) {
+    var level = params.options.level || 1;
+    var tag = "h" + level;
+    var hasTopLevelHeading = false;
+    shared.filterTokens(params, "heading_open", function forToken(token) {
+      if (token.tag === tag) {
+        if (hasTopLevelHeading) {
+          errors.addContext(token.lineNumber, token.line.trim());
+        } else if (token.lineNumber === 1) {
+          hasTopLevelHeading = true;
+        }
+      }
+    });
+  }
+};