Post properties video_view_count and video_duration

Closes #239.

instaloader/structures.py

@@ -284,6 +284,20 @@ class Post:
             return self._field('video_url')
         return None
 
+    @property
+    def video_view_count(self) -> Optional[int]:
+        """View count of the video, or None."""
+        if self.is_video:
+            return self._field('video_view_count')
+        return None
+
+    @property
+    def video_duration(self) -> Optional[float]:
+        """Duration of the video in seconds, or None."""
+        if self.is_video:
+            return self._field('video_duration')
+        return None
+
     @property
     def viewer_has_liked(self) -> Optional[bool]:
         """Whether the viewer has liked the post, or None if not logged in."""