Cmd-A to select everything (#51)

src/index.tsx

@@ -381,6 +381,7 @@ class App extends React.Component<{}, AppState> {
     if (event.key === "Escape") {
       clearSelection();
       drawScene();
+      event.preventDefault();
     } else if (event.key === "Backspace") {
       deleteSelectedElements();
       drawScene();
@@ -402,6 +403,12 @@ class App extends React.Component<{}, AppState> {
       });
       drawScene();
       event.preventDefault();
+    } else if (event.key === "a" && event.metaKey) {
+      elements.forEach(element => {
+        element.isSelected = true;
+      });
+      drawScene();
+      event.preventDefault();
     }
   };