minibar: add an option to make it easy to select png or svg output

for abstract synax trees, parse trees and word alignment diagrams.

src/www/minibar/minibar_online.js

@@ -10,6 +10,7 @@ var online_options={
 var minibar_options= {
   show_abstract: true,
   show_trees: true,
+  tree_img_format: "png", // or "svg"
   show_grouped_translations: false,
   default_source_language: "Eng",
 //feedback_url: "feedback.html",