Improve support for 9gag.com

[Rudloff]

Make sure you are using the latest version: run youtube-dl --version and ensure your version is 2018.06.04. 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 Issues with outdated version will be rejected.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 2018.06.04

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?

• 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: [u'-v', u'https://9gag.com/gag/aeM2g6W']
[debug] Encodings: locale UTF-8, fs UTF-8, out UTF-8, pref UTF-8
[debug] youtube-dl version 2018.06.04
[debug] Python version 2.7.13 (CPython) - Linux-4.9.0-6-amd64-x86_64-with-debian-9.4
[debug] exe versions: avconv 3.2.10-1, avprobe 3.2.10-1, ffmpeg 3.2.10-1, ffprobe 3.2.10-1, rtmpdump 2.4
[debug] Proxy map: {}
[generic] aeM2g6W: Requesting header
WARNING: Falling back on generic information extractor.
[generic] aeM2g6W: Downloading webpage
[generic] aeM2g6W: Extracting information
ERROR: Unsupported URL: https://9gag.com/gag/aeM2g6W
Traceback (most recent call last):
  File "./youtube-dl/youtube_dl/extractor/generic.py", line 2308, in _real_extract
    doc = compat_etree_fromstring(webpage.encode('utf-8'))
  File "./youtube-dl/youtube_dl/compat.py", line 2542, in compat_etree_fromstring
    doc = _XML(text, parser=etree.XMLParser(target=_TreeBuilder(element_factory=_element_factory)))
  File "./youtube-dl/youtube_dl/compat.py", line 2531, in _XML
    parser.feed(text)
  File "/usr/lib/python2.7/xml/etree/ElementTree.py", line 1653, in feed
    self._raiseerror(v)
  File "/usr/lib/python2.7/xml/etree/ElementTree.py", line 1517, in _raiseerror
    raise err
ParseError: not well-formed (invalid token): line 41, column 43
Traceback (most recent call last):
  File "./youtube-dl/youtube_dl/YoutubeDL.py", line 792, in extract_info
    ie_result = ie.extract(url)
  File "./youtube-dl/youtube_dl/extractor/common.py", line 500, in extract
    ie_result = self._real_extract(url)
  File "./youtube-dl/youtube_dl/extractor/generic.py", line 3263, in _real_extract
    raise UnsupportedError(url)
UnsupportedError: Unsupported URL: https://9gag.com/gag/aeM2g6W

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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://9gag.com/gag/aeM2g6W
• Single video: https://9gag.com/gag/aV3LxDP

Note that youtube-dl does not support sites dedicated to copyright infringement. In order for site support request to be accepted all provided example URLs should not violate any copyrights.

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Description of your issue, suggested solution and other information

The ninegag extractor does not seem to be triggered correctly for these videos.