extractors: update youku api

[iawia002]

the algorithm comes from @JayXon, many thanks

add a new parameter(refer) for Request

close #53

#35
#43

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@04a7198). Click here to learn what that means.
The diff coverage is 87.8%.

@@            Coverage Diff            @@
##             master      #79   +/-   ##
=========================================
  Coverage          ?   88.11%           
=========================================
  Files             ?       16           
  Lines             ?      867           
  Branches          ?        0           
=========================================
  Hits              ?      764           
  Misses            ?       83           
  Partials          ?       20

Impacted Files	Coverage Δ
utils/utils.go	64.28% <0%> (ø)
extractors/bcy.go	100% <100%> (ø)
extractors/facebook.go	100% <100%> (ø)
extractors/douyu.go	95.45% <100%> (ø)
extractors/youtube/youtube.go	95.65% <100%> (ø)
extractors/vimeo.go	100% <100%> (ø)
extractors/iqiyi.go	97.61% <100%> (ø)
extractors/tumblr.go	96.92% <100%> (ø)
extractors/youku.go	95.45% <100%> (ø)
extractors/douyin.go	100% <100%> (ø)
... and 3 more

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[iawia002]

Only 1080p works, others still got this problem (#43)