Use localStorage to cache images on client side

static/canvas.js

@@ -37,11 +37,11 @@ let get = function(url, cb) {
 }
 
 let getImage = function(id, cb) {
-  if (cachedImages[id])
-    cb(cachedImages[id]);
+  if (localStorage.getItem('img:' + id))
+    cb(localStorage.getItem('img:' + id));
   else {
     get('./imgs/' + id + '.svg', function(res) {
-      cachedImages[id] = res;
+      localStorage.setItem('img:' + id, res);
       cb(res);
     });
   }