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/firstLineH1.js

@@ -0,0 +1,32 @@
+module.exports = {
+  "name": "MD041",
+  "desc": "First line in file should be a top level header",
+  "tags": [ "headers" ],
+  "aliases": [ "first-line-h1" ],
+  "regexp": null,
+  "func": function MD041(params, errors) {
+    var level = params.options.level || 1;
+    var frontMatterTitle = params.options.front_matter_title;
+    var tag = "h" + level;
+    var frontMatterTitleRe =
+      new RegExp(frontMatterTitle || "^\\s*title:", "i");
+    params.tokens.every(function forToken(token, index) {
+      if (token.type === "heading_open") {
+        if (!((token.lineNumber === 1) || (index > 0)) ||
+            (token.tag !== tag)) {
+          errors.addContext(token.lineNumber, token.line);
+        }
+        return false;
+      } else if (token.type === "html_block") {
+        return true;
+      }
+      if (((frontMatterTitle !== undefined) && !frontMatterTitle) ||
+        !params.frontMatterLines.some(function forLine(line) {
+          return frontMatterTitleRe.test(line);
+        })) {
+        errors.addContext(token.lineNumber, token.line);
+      }
+      return false;
+    });
+  }
+};