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/headerIncrement.js

@@ -0,0 +1,20 @@
+var shared = require("../shared");
+
+module.exports = {
+"name": "MD001",
+"desc": "Header levels should only increment by one level at a time",
+"tags": [ "headers" ],
+"aliases": [ "header-increment" ],
+"regexp": null,
+"func": function MD001(params, errors) {
+  var prevLevel = 0;
+  shared.filterTokens(params, "heading_open", function forToken(token) {
+    var level = parseInt(token.tag.slice(1), 10);
+    if (prevLevel && (level > prevLevel)) {
+      errors.addDetailIf(token.lineNumber,
+        "h" + (prevLevel + 1), "h" + level);
+      }
+      prevLevel = level;
+    });
+  }
+};