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/blanksAroundHeaders.js

@@ -0,0 +1,34 @@
+module.exports = {
+  "name": "MD022",
+  "desc": "Headers should be surrounded by blank lines",
+  "tags": [ "headers", "blank_lines" ],
+  "aliases": [ "blanks-around-headers" ],
+  "regexp": null,
+  "func": function MD022(params, errors) {
+    var prevHeadingLineNumber = 0;
+    var prevMaxLineIndex = -1;
+    var needBlankLine = false;
+    params.tokens.forEach(function forToken(token) {
+      if (token.type === "heading_open") {
+        if ((token.map[0] - prevMaxLineIndex) === 0) {
+          errors.addContext(token.lineNumber, token.line.trim());
+        }
+      } else if (token.type === "heading_close") {
+        needBlankLine = true;
+      }
+      if (token.map) {
+        if (needBlankLine) {
+          if ((token.map[0] - prevMaxLineIndex) === 0) {
+            errors.addContext(prevHeadingLineNumber,
+              params.lines[prevHeadingLineNumber - 1].trim());
+          }
+          needBlankLine = false;
+        }
+        prevMaxLineIndex = Math.max(prevMaxLineIndex, token.map[1]);
+      }
+      if (token.type === "heading_open") {
+        prevHeadingLineNumber = token.lineNumber;
+      }
+    });
+  }
+};