added identifier option to search

CalibreWebCompanion/library/templates/search.html

@@ -8,6 +8,8 @@
     <input id="title" type="text" name="title" value="">
     <label for="author">Author: </label>
     <input id="author" type="text" name="author" value="">
+    <label for="author">Identifier: </label>
+    <input id="identifier" type="text" name="identifier" value="">
     <button class="waves-effect waves-light btn green accent-4" type="submit">search</button>
 </form>
 </div>