[Elib] Add new extractor

[vitkyrka]

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What is the purpose of your pull request?

• Bug fix
• Improvement
• New extractor
• New feature

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Description of your pull request and other information

Add support for Elib, an audio book service used by many Swedish libraries. It uses MP3 delivered via HLS so a small fix is required in FFmpegFD to create an MP3 instead of attempting to make a video.

[dstftw]

PS C:\dev\youtube-dl\master> py -3.5 .\youtube_dl\__main__.py -v https://delivery-32.elib.se/Library/123a4d5a-1e3a-2412-84ae-4a36134e0ac1
[debug] System config: []
[debug] User config: []
[debug] Custom config: []
[debug] Command-line args: ['-v', 'https://delivery-32.elib.se/Library/123a4d5a-1e3a-2412-84ae-4a36134e0ac1']
[debug] Encodings: locale cp1251, fs mbcs, out cp866, pref cp1251
[debug] youtube-dl version 2017.01.05
[debug] Git HEAD: 3d315e6
[debug] Python version 3.5.2 - Windows-10-10.0.10240-SP0
[debug] exe versions: ffmpeg N-81192-g04da20e, ffprobe N-81192-g04da20e, rtmpdump 2.4
[debug] Proxy map: {}
[Elib] 123a4d5a-1e3a-2412-84ae-4a36134e0ac1: Downloading JSON metadata
ERROR: Unable to download JSON metadata: HTTP Error 401: Invalid ticket 123a4d5a-1e3a-2412-84ae-4a36134e0ac1 (caused by <HTTPError 401: 'Invalid ticket 123a4d5a-1e3a-2412-84ae-4a36134e0ac1'>); please report this issue on https://yt-dl.org/bug . Make sure you are using the latest version; see  https://yt-dl.org/update  on how to update. Be sure to call youtube-dl with the --verbose flag and include its complete output.
  File "C:\dev\youtube-dl\master\youtube_dl\extractor\common.py", line 406, in _request_webpage
    return self._downloader.urlopen(url_or_request)
  File "C:\dev\youtube-dl\master\youtube_dl\YoutubeDL.py", line 2001, in urlopen
    return self._opener.open(req, timeout=self._socket_timeout)
  File "C:\Python\Python35\lib\urllib\request.py", line 472, in open
    response = meth(req, response)
  File "C:\Python\Python35\lib\urllib\request.py", line 582, in http_response
    'http', request, response, code, msg, hdrs)
  File "C:\Python\Python35\lib\urllib\request.py", line 510, in error
    return self._call_chain(*args)
  File "C:\Python\Python35\lib\urllib\request.py", line 444, in _call_chain
    result = func(*args)
  File "C:\Python\Python35\lib\urllib\request.py", line 590, in http_error_default
    raise HTTPError(req.full_url, code, msg, hdrs, fp)

[vitkyrka]

That's expected, the URL is fake as the comment says. This is a subscription service and the URLs identify the loan session from the library so I can't include a real URL in the test.

[dstftw]

Then you must provide valid account credentials and clear instructions on how this can be tested. Nobody of collaborators have one.