Align left/center/right multiline messages

src/mermaidAPI.js

@@ -229,6 +229,14 @@ const config = {
      */
     messageMargin: 35,
 
+    /**
+     * Multiline message alignment. Possible values are:
+     *   * left
+     *   * center **default**
+     *   * right
+     */
+    messageAlign: 'center',
+
     /**
      * mirror actors under diagram.
      * **Default value true**.
@@ -809,6 +817,7 @@ export default mermaidAPI;
  *       boxTextMargin:5,
  *       noteMargin:10,
  *       messageMargin:35,
+ *       messageAlign:'center',
  *       mirrorActors:true,
  *       bottomMarginAdj:1,
  *       useMaxWidth:true,