Add video section to the website.

This is the work for this issue: #43 (closed)

Preview:

Note: In the preview, I add some videos from google code-in 2017 to test because I haven't done any video task.

How to add video to the website:

• First upload your video image to assets/img/videos.
• Then add your video to _data/videos.yml file like this:

 - url: https://www.youtube.com/watch?v=P4TqM8Qpq7A
   title: Fossasia Summit Promotional Video
   img: saarthakvid.jpg
   author_name: Saarthak Chaturvedi