Add labels to relationships (needs further tidy up)

src/mermaidAPI.js

@@ -351,6 +351,12 @@ const config = {
    * The object containing configurations specific for entity relationship diagrams
    */
   er: {
+    /**
+     * Directional bias for layout of entities. Can be either 'TB', 'BT', 'LR', or 'RL', 
+     * where T = top, B = bottom, L = left, and R = right. 
+     */
+    layoutDirection: 'TB',
+
     /**
      * The mimimum width of an entity box
      */