[imgur] add support for videos with audio

[caioalonso]

• I've verified and I assure that I'm running youtube-dl 2018.05.26

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 ```):

$ youtube-dl -v "https://imgur.com/t/unmuted/FVyxO32"                                                                                                                                              5s 381ms
[debug] System config: []
[debug] User config: []
[debug] Custom config: []
[debug] Command-line args: ['-v', 'https://imgur.com/t/unmuted/FVyxO32']
[debug] Encodings: locale UTF-8, fs utf-8, out UTF-8, pref UTF-8
[debug] youtube-dl version 2018.05.26
[debug] Python version 3.5.2 (CPython) - Linux-4.13.0-43-generic-x86_64-with-LinuxMint-18.3-sylvia
[debug] exe versions: ffmpeg 2.8.14-0ubuntu0.16.04.1, ffprobe 2.8.14-0ubuntu0.16.04.1
[debug] Proxy map: {}
[generic] FVyxO32: Requesting header
WARNING: Falling back on generic information extractor.
[generic] FVyxO32: Downloading webpage
[generic] FVyxO32: Extracting information
ERROR: Unsupported URL: https://imgur.com/t/unmuted/FVyxO32
Traceback (most recent call last):
  File "/usr/local/lib/python3.5/dist-packages/youtube_dl/YoutubeDL.py", line 792, in extract_info
    ie_result = ie.extract(url)
  File "/usr/local/lib/python3.5/dist-packages/youtube_dl/extractor/common.py", line 500, in extract
    ie_result = self._real_extract(url)
  File "/usr/local/lib/python3.5/dist-packages/youtube_dl/extractor/generic.py", line 3240, in _real_extract
    raise UnsupportedError(url)
youtube_dl.utils.UnsupportedError: Unsupported URL: https://imgur.com/t/unmuted/FVyxO32

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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://imgur.com/t/unmuted/FVyxO32

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Description of your issue, suggested solution and other information

Imgur recently added support for videos with audio: https://blog.imgur.com/2018/05/29/upload-video-with-sound-on-imgur-for-ios/