Move each rule implementation into its own file (fixes #83).

lib/md023.js

@@ -0,0 +1,21 @@
+// @ts-check
+
+"use strict";
+
+var shared = require("./shared");
+
+var spaceBeforeHeaderRe = /^\s+\S/;
+
+module.exports = {
+  "names": [ "MD023", "header-start-left" ],
+  "description": "Headers must start at the beginning of the line",
+  "tags": [ "headers", "spaces" ],
+  "function": function MD023(params, onError) {
+    shared.filterTokens(params, "heading_open", function forToken(token) {
+      if (spaceBeforeHeaderRe.test(token.line)) {
+        shared.addErrorContext(onError, token.lineNumber, token.line, null,
+          null, shared.rangeFromRegExp(token.line, spaceBeforeHeaderRe));
+      }
+    });
+  }
+};