Add version 2.0.0 to the dropdown menu

doc/_templates/layout.html

@@ -53,6 +53,7 @@
               <a href="#" class="dropdown-toggle" data-toggle="dropdown" role="button"
                  aria-expanded="false">{{ version }} <span class="caret"></span></a>
               <ul class="dropdown-menu" role="menu">
+                <li><a href="http://cppformat.github.io/2.0.0/">2.0.0</a></li>
                 <li><a href="http://cppformat.github.io/1.1.0/">1.1.0</a></li>
                 <li><a href="http://cppformat.github.io/1.0.0/">1.0.0</a></li>
               </ul>