Add option "--timing" to mesure gitbook performances

lib/parse/parseBook.js

@@ -1,4 +1,5 @@
 var Promise = require('../utils/promise');
+var timing = require('../utils/timing');
 var Book = require('../models/book');
 
 var parseIgnore = require('./parseIgnore');
@@ -57,7 +58,9 @@ function parseMultilingualBook(book) {
     @return {Promise<Book>}
 */
 function parseBook(book) {
-    return Promise(book)
+    return timing.measure(
+        'parse.book',
+        Promise(book)
         .then(parseIgnore)
         .then(parseConfig)
         .then(parseLanguages)
@@ -67,7 +70,8 @@ function parseBook(book) {
             } else {
                 return parseBookContent(resultBook);
             }
-        });
+        })
+    );
 }
 
 module.exports = parseBook;