[vk] Add support for sibnet embeds

[kotofond]

Make sure you are using the latest version: run youtube-dl --version and ensure your version is 2016.05.10. 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 Issues with outdated version will be rejected.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 2016.05.10

Before submitting an issue make sure you have:

• 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

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D:\>youtube-dl --verbose https://vk.com/video-19038459_456239609
[debug] System config: []
[debug] User config: []
[debug] Command-line args: ['--verbose', 'https://vk.com/video-19038459_456239609']
[debug] Encodings: locale cp1251, fs mbcs, out cp866, pref cp1251
[debug] youtube-dl version 2016.05.10
[debug] Python version 3.5.1 - Windows-10-10.0.10586-SP0
[debug] exe versions: ffmpeg N-78358-g674cc26, ffprobe N-78358-g674cc26
[debug] Proxy map: {}
[vk] -19038459_456239609: Downloading webpage
ERROR: Unable to extract vars; 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.
Traceback (most recent call last):
  File "c:\python35\lib\site-packages\youtube_dl\YoutubeDL.py", line 676, in extract_info
    ie_result = ie.extract(url)
  File "c:\python35\lib\site-packages\youtube_dl\extractor\common.py", line 341, in extract
    return self._real_extract(url)
  File "c:\python35\lib\site-packages\youtube_dl\extractor\vk.py", line 297, in _real_extract
    data_json = self._search_regex(r'var\s+vars\s*=\s*({.+?});', info_page, 'vars')
  File "c:\python35\lib\site-packages\youtube_dl\extractor\common.py", line 644, in _search_regex
    raise RegexNotFoundError('Unable to extract %s' % _name)
youtube_dl.utils.RegexNotFoundError: Unable to extract vars; 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.

[kotofond]

Another example: http://vk.com/video-39393647_171962303