main: rename Download to Extract

[iawia002]

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[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@5ebfebe). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master     #350   +/-   ##
=========================================
  Coverage          ?   71.83%           
=========================================
  Files             ?       29           
  Lines             ?     2084           
  Branches          ?        0           
=========================================
  Hits              ?     1497           
  Misses            ?      424           
  Partials          ?      163

Impacted Files	Coverage Δ
extractors/douyu/douyu.go	78.94% <100%> (ø)
extractors/douyin/douyin.go	85.18% <100%> (ø)
extractors/pixivision/pixivision.go	80% <100%> (ø)
extractors/iqiyi/iqiyi.go	85.58% <100%> (ø)
extractors/facebook/facebook.go	86.66% <100%> (ø)
extractors/tumblr/tumblr.go	78.31% <100%> (ø)
extractors/twitter/twitter.go	68.91% <100%> (ø)
extractors/bcy/bcy.go	80.48% <100%> (ø)
extractors/youtube/youtube.go	82.85% <100%> (ø)
extractors/miaopai/miaopai.go	86.66% <100%> (ø)
... and 10 more

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