Add support for brtn.cn

[yanderemoe]

Make sure you are using the latest version: run youtube-dl --version and ensure your version is 2018.12.17. If it's not, read this FAQ entry and update. Issues with outdated version will be rejected.

• I've verified and I assure that I'm running youtube-dl 2018.12.17

Before submitting an issue make sure you have:

• At least skimmed through the README, most notably the FAQ and BUGS sections
• Searched the bugtracker for similar issues including closed ones
• Checked that provided video/audio/playlist URLs (if any) are alive and playable in a browser

What is the purpose of your issue?

• Site support request (request for adding support for a new site)

If the purpose of this issue is a site support request please provide all kinds of example URLs support for which should be included (replace following example URLs by yours):

• Single video: http://www.brtn.cn/news/f3v2ngkgj0s8quqtersvjpehoo9
• JSON link file: http://api.btime.com/video/play?from=pc&id=f3v2ngkgj0s8quqtersvjpehoo9
  (the JSON file contains all of the URL that needed for downloading)

{
  "errno": 0,
  "data": {
    "play_status": 2,
    "video_stream": [
      {
        "stream_url": "http://mp4.v.btime.com/vod-ugc-btime-q2-bj/182931917_2-1545291366-7cf36e52-52ac-c58f.mp4?time=1545851426&sign=b8e38cf2d0b557b0c575d471871ec882",
        "stream_rate": 1000,
        "stream_vbt": "高清",
        "stream_type": 7
      },
      {
        "stream_url": "http://mp4.v.btime.com/vod-ugc-btime-q2-bj/182931917_4-1545291366-bf9626ef-e479-cefe.mp4?time=1545851426&sign=cced0b3ac95fe792a0d7ce90122001be",
        "stream_rate": 800,
        "stream_vbt": "标清",
        "stream_type": 7
      }
    ],
    "ts": 1545851426,
    "gid": "f3v2ngkgj0s8quqtersvjpehoo9",
    "utoken": "",
    "video_source_type": 3,
    "duration": "00:47:15"
  }
}

There should be no copyright problems since it is a goverment-owned public television network in Beijing.
https://en.wikipedia.org/wiki/Beijing_Media_Network