special-case inject prototype for FB and amazon

Some sites inject overlays over their videos to intercept clicks and
provide own on-screen controls, etc. Unfortunately this makes the VC
controls inaccessible and without any generic workaround... well, short
of relying on keyboard shortcuts.

This is an experiment for special-casing FB and Amazon
- FB we modify stacking context of the parent
- Amazon we inject the controller one level higher, above the overlay

The gotcha here is that this type of behavior is not uncommon, and
special-casing each and every origin like this is not long-term
sustainable. If this sticks.. we'll have to be very selective
about which sites we enable this for.

Closes #210
Closes #134

inject.js

@@ -136,12 +136,26 @@ chrome.extension.sendMessage({}, function(response) {
       var fragment = document.createDocumentFragment();
       fragment.appendChild(wrapper);
 
-      // Note: when triggered via a MutationRecord, it's possible that the
-      // target is not the immediate parent. This appends the controller as
-      // the first element of the target, which may not be the parent.
-      this.parent.insertBefore(fragment, this.parent.firstChild);
       this.video.classList.add('vsc-initialized');
       this.video.dataset['vscid'] = this.id;
+
+      switch (location.hostname) {
+        case 'www.amazon.com':
+          // insert before parent to bypass overlay
+          this.parent.parentElement.insertBefore(fragment, this.parent);
+          break;
+
+        case 'www.facebook.com':
+          // set stacking context to same as parent's parent.
+          // + default fallthrough
+          this.parent.style.zIndex = 'auto';
+
+        default:
+          // Note: when triggered via a MutationRecord, it's possible that the
+          // target is not the immediate parent. This appends the controller as
+          // the first element of the target, which may not be the parent.
+          this.parent.insertBefore(fragment, this.parent.firstChild);
+      }
     }
   }