[youtube] Improve extraction of video duration

[cristalix]

Hi,
suggestion how to improve extraction of video duration. Note: I'm nod good in python :) This is current code how ytdl extract duration info about video, isn't it:

if 'length_seconds' not in video_info:
    self._downloader.report_warning('unable to extract video duration')
    video_duration = None
else:
    video_duration = int(compat_urllib_parse_unquote_plus(video_info['length_seconds'][0]))

There is duration info also in https://www.youtube.com/watch?v=GcjcbqQAjfk page in header under property:

<meta itemprop="duration" content="PT6M41S">

It's in some YT, let's say, specific format :)