[vk] Add support for live streams.

[ghost]

• I've verified and I assure that I'm running youtube-dl 2016.09.27

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• At least skimmed through README and most notably 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

$ y https://vk.com/video-387766_456242765
[vk] -387766_456242765: Downloading webpage
Traceback (most recent call last):
  File "/usr/local/bin/youtube-dl", line 11, in <module>
    load_entry_point('youtube-dl==2016.9.27', 'console_scripts', 'youtube-dl')()
  File "/usr/local/lib/python3.6/dist-packages/youtube_dl/__init__.py", line 449, in main
    _real_main(argv)
  File "/usr/local/lib/python3.6/dist-packages/youtube_dl/__init__.py", line 439, in _real_main
    retcode = ydl.download(all_urls)
  File "/usr/local/lib/python3.6/dist-packages/youtube_dl/YoutubeDL.py", line 1791, in download
    url, force_generic_extractor=self.params.get('force_generic_extractor', False))
  File "/usr/local/lib/python3.6/dist-packages/youtube_dl/YoutubeDL.py", line 705, in extract_info
    return self.process_ie_result(ie_result, download, extra_info)
  File "/usr/local/lib/python3.6/dist-packages/youtube_dl/YoutubeDL.py", line 751, in process_ie_result
    return self.process_video_result(ie_result, download=download)
  File "/usr/local/lib/python3.6/dist-packages/youtube_dl/YoutubeDL.py", line 1372, in process_video_result
    format['http_headers'] = self._calc_headers(full_format_info)
  File "/usr/local/lib/python3.6/dist-packages/youtube_dl/YoutubeDL.py", line 1227, in _calc_headers
    cookies = self._calc_cookies(info_dict)
  File "/usr/local/lib/python3.6/dist-packages/youtube_dl/YoutubeDL.py", line 1234, in _calc_cookies
    pr = sanitized_Request(info_dict['url'])
  File "/usr/local/lib/python3.6/dist-packages/youtube_dl/utils.py", line 508, in sanitized_Request
    return compat_urllib_request.Request(sanitize_url(url), *args, **kwargs)
  File "/usr/lib/python3.6/urllib/request.py", line 329, in __init__
    self.full_url = url
  File "/usr/lib/python3.6/urllib/request.py", line 355, in full_url
    self._parse()
  File "/usr/lib/python3.6/urllib/request.py", line 384, in _parse
    raise ValueError("unknown url type: %r" % self.full_url)
ValueError: unknown url type: '-387766_456242765'

https://vk.com/video-387766_456242765