[Wistia] Add support to download captions and thumbnail

[witedragen]

Checklist

• I'm reporting a broken site support
• I've verified that I'm running youtube-dl version 2019.09.28
• I've checked that all provided URLs are alive and playable in a browser
• I've checked that all URLs and arguments with special characters are properly quoted or escaped
• I've searched the bugtracker for similar issues including closed ones

Verbose log

[debug] System config: []
[debug] User config: []
[debug] Custom config: []
[debug] Command-line args: ['--ignore-config', '-i', '--skip-download', '--all-subs', '--write-sub', '--external-downloader=aria2c', '--verbose', 'https://cisco-netacad.wistia.com/medias/wsgq6aoelv']
[debug] Encodings: locale UTF-8, fs utf-8, out UTF-8, pref UTF-8
[debug] youtube-dl version 2019.09.28
[debug] Python version 3.7.4 (CPython) - Darwin-17.7.0-x86_64-i386-64bit
[debug] exe versions: ffmpeg 4.1.4, ffprobe 4.1.4, phantomjs 2.1.1, rtmpdump 2.4
[debug] Proxy map: {}
[generic] wsgq6aoelv: Requesting header
WARNING: Falling back on generic information extractor.
[generic] wsgq6aoelv: Downloading webpage
[generic] wsgq6aoelv: Extracting information
[Wistia] wsgq6aoelv: Downloading JSON metadata
[debug] Default format spec: bestvideo+bestaudio/best

Description

When using both --write-sub and --write-auto-sub I am given no subtitles when the link provided definitely has them. Wasn't 100% sure if the video had a thumbnail but I noticed one was not downloaded so I put that in the request, too.

URL: https://cisco-netacad.wistia.com/medias/wsgq6aoelv