add npm-cmd: prepare & fix videoTitle toggle bug.

package.json

     "i:demo": "npm run install:demo",
     "test": "jest --env=jsdom",
     "test:watch": "npm run test -- --watch",
+    "prepare": "npm run build",
     "build": "webpack --display-error-details",
     "build:demo": "cd demo && webpack",
     "deploy:demo": "node scripts/deploy:demo.js",

src/video/api.js

  * set. To be primarily used in `mapVideoElToProps`.
  */
 export const togglePause = (videoEl, { paused }) => {
-    if (paused) {
-        videoEl.play();
-        document.querySelector("#videoTitle").style.display = 'none';
-    } else {
-        videoEl.pause();
-        document.querySelector("#videoTitle").style.display = '';
+    const videoTitleEl = document.getElementById("videoTitle")
+    if (videoTitleEl) {
+        if (paused) {
+            videoEl.play();
+            videoTitleEl.style.display = 'none';
+        } else {
+            videoEl.pause();
+            videoTitleEl.style.display = '';
+        }
     }
 };