Feature request: Add uploader URL to output dict of YouTube videos

[zhuyifei1999]

As shown in #8712, there is no mentioning of uploader's url in output dict of YouTube videos.

Why is this needed? For complying with the attribution requirements, an explicit url is sometimes required to uniquely identify an uploader (see downstream bug). While I see there's 'uploader_id' in output dict, some ambiguity exist that make programmatic determining of the url difficult:

>>> dl.extract_info('https://www.youtube.com/watch?v=eQcmzGIKrzg', download=False)['uploader_id'] # uploader url is https://www.youtube.com/channel/UCH1dpzjCEiGAt8CXkryhkZg
WARNING: Parameter outtmpl is bytes, but should be a unicode string. Put  from __future__ import unicode_literals  at the top of your code file or consider switching to Python 3.x.
[debug] Encodings: locale UTF-8, fs UTF-8, out UTF-8, pref UTF-8
[debug] youtube-dl version 2016.03.01
[debug] Python version 2.7.6 - Linux-3.19.0-49-generic-x86_64-with-Ubuntu-14.04-trusty
[debug] exe versions: avconv 9.18-6, avprobe 9.18-6
[debug] Proxy map: {}
[youtube] eQcmzGIKrzg: Downloading webpage
[youtube] eQcmzGIKrzg: Downloading video info webpage
[youtube] eQcmzGIKrzg: Extracting video information
[youtube] eQcmzGIKrzg: Downloading MPD manifest
u'UCH1dpzjCEiGAt8CXkryhkZg'
>>> dl.extract_info('https://www.youtube.com/watch?v=yk-IMpolG_Y', download=False)['uploader_id'] # uploader url is https://www.youtube.com/user/lauraflanders
WARNING: Parameter outtmpl is bytes, but should be a unicode string. Put  from __future__ import unicode_literals  at the top of your code file or consider switching to Python 3.x.
[debug] Encodings: locale UTF-8, fs UTF-8, out UTF-8, pref UTF-8
[debug] youtube-dl version 2016.03.01
[debug] Python version 2.7.6 - Linux-3.19.0-49-generic-x86_64-with-Ubuntu-14.04-trusty
[debug] exe versions: avconv 9.18-6, avprobe 9.18-6
[debug] Proxy map: {}
[youtube] yk-IMpolG_Y: Downloading webpage
[youtube] yk-IMpolG_Y: Downloading video info webpage
[youtube] yk-IMpolG_Y: Extracting video information
[youtube] yk-IMpolG_Y: Downloading MPD manifest
u'lauraflanders'

In the first case, https://www.youtube.com/channel/ should be prepended to get the correct url, while in the second case https://www.youtube.com/user/ should be prepended instead.