Add Box support in Sequence Diagrams

demos/sequence.html

@@ -128,6 +128,22 @@
     </pre>
     <hr />
 
+    <pre class="mermaid">
+      sequenceDiagram
+      box lightgreen Alice & John
+      participant A
+      participant J
+      end
+      box Another Group very very long description not wrapped
+      participant B
+      end
+      A->>J: Hello John, how are you?
+      J->>A: Great!
+      A->>B: Hello Bob, how are you ?
+      </pre
+    >
+    <hr />
+
     <script src="./mermaid.js"></script>
     <script>
       mermaid.initialize({