Add support for authoring custom rules.

test/rules/any-blockquote.js

@@ -0,0 +1,22 @@
+// @ts-check
+
+"use strict";
+
+module.exports = {
+  "names": [ "any-blockquote" ],
+  "description": "Rule that reports an error for any blockquote",
+  "tags": [ "test" ],
+  "function": function rule(params, onError) {
+    params.tokens.filter(function filterToken(token) {
+      return token.type === "blockquote_open";
+    }).forEach(function forToken(blockquote) {
+      var lines = blockquote.map[1] - blockquote.map[0];
+      onError({
+        "lineNumber": blockquote.lineNumber,
+        "detail": "Blockquote spans " + lines + " line(s).",
+        "context": blockquote.line.substr(0, 7),
+        "range": null
+      });
+    });
+  }
+};