Cache images in javascript

      close #3

canvas.js

+let cachedImages = [];
+
 let canvas = document.getElementById('output');
 let ctx = canvas.getContext('2d');
 
@@ -15,6 +17,17 @@ let get = function(url, cb) {
   xmlHttp.send(null);
 }
 
+let getImage = function(id, cb) {
+  if (cachedImages[id])
+    cb(cachedImages[id]);
+  else {
+    get('./imgs/' + id + '.svg', function(res) {
+      cachedImages[id] = res;
+      cb(res);
+    });
+  }
+}
+
 let draw = function() {
   let bgColor = document.querySelector('.colorpicker input[type="radio"][name="bg-color"]:checked').value;
   let fgColor = document.querySelector('.colorpicker input[type="radio"][name="fg-color"]:checked').value;
@@ -67,7 +80,7 @@ let draw = function() {
   ctx.fillText('La Quadrature du Net !', offset, height * 0.9);
 
   //Draw picto
-  get('./imgs/' + icon + '.svg', function(res) {
+  getImage(icon, function(res) {
     let img = new Image();
     let imgXml = res.replace(/#FFD201/g, fgColor);
     img.src = 'data:image/svg+xml;base64,' + btoa(imgXml);