Add basic infrastructure, MD031, and test.

lib/rules.js

@@ -0,0 +1,30 @@
+"use strict";
+
+function padAndTrim(lines) {
+  return [].concat(
+    "",
+    lines.map(function(line) {
+      return line.trim();
+    }),
+    "");
+}
+
+module.exports = {
+  MD031: function(lines) {
+    // Some parsers have trouble detecting fenced code blocks without
+    // surrounding whitespace, so examine the lines directly.
+    lines = padAndTrim(lines);
+    var errors = [];
+    var inCode = false;
+    lines.forEach(function(line, lineNum) {
+      if (line.match(/^(```|~~~)/)) {
+        inCode = !inCode;
+        if ((inCode && lines[lineNum - 1].length) ||
+            (!inCode && lines[lineNum + 1].length)) {
+          errors.push(lineNum);
+        }
+      }
+    });
+    return errors;
+  }
+};