downloader: ⬆️ aria2

[iawia002]

continue #284

cc @hguandl, I have not changed the main logic, just move the part to the right place.

[codecov]

Codecov Report

Merging #286 into master will decrease coverage by 3.6%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #286      +/-   ##
==========================================
- Coverage   73.26%   69.66%   -3.61%     
==========================================
  Files          26       26              
  Lines        1919     1925       +6     
==========================================
- Hits         1406     1341      -65     
- Misses        358      433      +75     
+ Partials      155      151       -4

Impacted Files	Coverage Δ
downloader/downloader.go	43.82% <0%> (-1.08%)	⬇️
extractors/twitter/twitter.go	16.43% <0%> (-52.06%)	⬇️
extractors/youku/youku.go	66.93% <0%> (-21.78%)	⬇️

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