feat: support download npm package

[atian25]

eggjs/egg#456

[mention-bot]

@atian25, thanks for your PR! By analyzing the history of the files in this pull request, we identified @jtyjty99999 to be a potential reviewer.

[codecov]

Codecov Report

Merging #22 into master will increase coverage by 0.31%.
The diff coverage is 90%.

@@            Coverage Diff             @@
##           master      #22      +/-   ##
==========================================
+ Coverage   81.25%   81.56%   +0.31%     
==========================================
  Files           1        1              
  Lines         176      179       +3     
==========================================
+ Hits          143      146       +3     
  Misses         33       33

Impacted Files	Coverage Δ
lib/init_command.js	81.56% <90%> (+0.31%)	✅

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[atian25]

1.8.0