Add MD004 with tests, support disabling rules.

lib/rules.js

@@ -14,6 +14,19 @@ function headingStyleFrom(token, lines) {
   return "setext";
 }
 
+function unorderedListStyleFrom(token, lines) {
+  switch (lines[token.lines[0]].trim().substr(0, 1)) {
+    case "*":
+      return "asterisk";
+    case "-":
+      return "dash";
+    case "+":
+      return "plus";
+    default:
+      return null;
+  }
+}
+
 function padAndTrim(lines) {
   return [].concat(
     "",
@@ -75,6 +88,25 @@ module.exports = [
     }
   },
 
+  {
+    "name": "MD004",
+    "desc": "Unordered list style",
+    "func": function MD004(params, errors) {
+      var style = params.options.style || "consistent";
+      var listItems = params.tokens.filter(function filterToken(token) {
+        return (token.type === "list_item_open");
+      });
+      if ((style === "consistent") && listItems.length) {
+        style = unorderedListStyleFrom(listItems[0], params.lines);
+      }
+      listItems.forEach(function forToken(token) {
+        if (unorderedListStyleFrom(token, params.lines) !== style) {
+          errors.push(lineNumberFrom(token));
+        }
+      });
+    }
+  },
+
   {
     "name": "MD031",
     "desc": "Fenced code blocks should be surrounded by blank lines",