Add "Videos" as supported platform

[MacDhughaill]

I discovered that in addition to the familiar "games, ebooks, audio" platforms, there is now a "video" platform. By default, hb-downloader does not list or download anything from the video platform. A simple change like below to hb-downloader-settings.yaml does successfully download the videos when a plain command of hb-downloader.py download is ran, though, so it seems support should be easy to add?

download-platforms:
  audio: True
  ebook: True
  windows: True
  mac: True
  linux: True
  android: True
  asmjs: False
  video: True

[MayeulC]

Indeed, support should be pretty easy to add, but I don't think I have any bundle to test this with. I'll look into it anyways; hopefully under one month.