[reddit] Add support for reddit's own video host

[goggle]

Make sure you are using the latest version: run youtube-dl --version and ensure your version is 2017.08.06. 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 Issues with outdated version will be rejected.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 2017.08.06

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

What is the purpose of your issue?

• Bug report (encountered problems with youtube-dl)
• Site support request (request for adding support for a new site)
• Feature request (request for a new functionality)
• Question
• Other

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If the purpose of this issue is a bug report, site support request or you are not completely sure provide the full verbose output as follows:

Add the -v flag to your command line you run youtube-dl with (youtube-dl -v <your command line>), copy the whole output and insert it here. It should look similar to one below (replace it with your log inserted between triple ```):

[debug] System config: []
[debug] User config: []
[debug] Custom config: []
[debug] Command-line args: ['-v', 'https://www.reddit.com/r/videos/comments/6rrwyj/that_small_heart_attack/']
[debug] Encodings: locale UTF-8, fs utf-8, out UTF-8, pref UTF-8
[debug] youtube-dl version 2017.08.06
[debug] Python version 3.6.2 - Linux-4.12.4-1-ARCH-x86_64-with-arch
[debug] exe versions: ffmpeg 3.3.3, ffprobe 3.3.3, rtmpdump 2.4
[debug] Proxy map: {}
[generic] that_small_heart_attack: Requesting header
WARNING: Falling back on generic information extractor.
[generic] that_small_heart_attack: Downloading webpage
[generic] that_small_heart_attack: Extracting information
ERROR: Unsupported URL: https://www.reddit.com/r/videos/comments/6rrwyj/that_small_heart_attack/
Traceback (most recent call last):
  File "/usr/lib/python3.6/site-packages/youtube_dl/YoutubeDL.py", line 776, in extract_info
    ie_result = ie.extract(url)
  File "/usr/lib/python3.6/site-packages/youtube_dl/extractor/common.py", line 433, in extract
    ie_result = self._real_extract(url)
  File "/usr/lib/python3.6/site-packages/youtube_dl/extractor/generic.py", line 2944, in _real_extract
    raise UnsupportedError(url)
youtube_dl.utils.UnsupportedError: Unsupported URL: https://www.reddit.com/r/videos/comments/6rrwyj/that_small_heart_attack/

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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: https://www.reddit.com/r/videos/comments/6rrwyj/that_small_heart_attack/ (or https://v.redd.it/zv89llsvexdz)
• Single video: https://www.reddit.com/r/holdmybeer/comments/6rn3e4/hmb_while_i_put_another_bottle_through_wood/
• Single video (without audio): https://www.reddit.com/r/NatureGifs/comments/6s0doa/bumblebee_lining_itself_up_to_land_on_a_swaying/

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Description of your issue, suggested solution and other information

Reddit has begun to host videos on its own on v.redd.it. It seems as they support to upload GIFs too and provide them as videos without audio through v.redd.it (if I understood this part correctly). It would be nice to have youtube-dl support for these videos.