Add MD023, MD024, MD025 with tests.

lib/rules.js

@@ -375,6 +375,59 @@ module.exports = [
     }
   },
 
+  {
+    "name": "MD023",
+    "desc": "Headers must start at the beginning of the line",
+    "func": function MD023(params, errors) {
+      filterTokens(params.tokens, "heading_open")
+        .forEach(function forToken(token) {
+          if (/^\s/.test(token.line)) {
+            errors.push(token.lineNumber);
+          }
+        });
+    }
+  },
+
+  {
+    "name": "MD024",
+    "desc": "Multiple headers with the same content",
+    "func": function MD024(params, errors) {
+      var content = [];
+      var inHeading = false;
+      params.tokens.forEach(function forToken(token) {
+        if (token.type === "heading_open") {
+          inHeading = true;
+        } else if (token.type === "heading_close") {
+          inHeading = false;
+        } else if ((token.type === "inline") && inHeading) {
+          if (content.indexOf(token.content) === -1) {
+            content.push(token.content);
+          } else {
+            errors.push(token.lineNumber);
+          }
+        }
+      });
+    }
+  },
+
+  {
+    "name": "MD025",
+    "desc": "Multiple top level headers in the same document",
+    "func": function MD025(params, errors) {
+      var hasTopLevelHeading = false;
+      filterTokens(params.tokens, "heading_open")
+        .forEach(function forToken(token) {
+          if (token.hLevel === 1) {
+            if (hasTopLevelHeading) {
+              errors.push(token.lineNumber);
+            } else if (token.lineNumber === 1) {
+              hasTopLevelHeading = true;
+            }
+          }
+        });
+    }
+  },
+
   {
     "name": "MD028",
     "desc": "Blank line inside blockquote",