Add support for multi-languages books in JSON format

lib/generate/json/index.js

@@ -40,6 +40,23 @@ Generator.prototype.convertFile = function(content, input) {
     });
 };
 
+// Generate languages index
+Generator.prototype.langsIndex = function(langs) {
+    var that = this;
+
+    var json = {
+        langs: langs.list
+    };
+
+    return Q()
+    .then(function() {
+        return fs.writeFile(
+            path.join(that.options.output, "langs.json"),
+            JSON.stringify(json, null, 4)
+        );
+    });
+};
+
 Generator.prototype.finish = function() {
     // ignore
 };