Support external subcommand like git-*

[k1LoW]

No description provided.

[codecov]

Codecov Report

Merging #204 into master will decrease coverage by 0.94%.
The diff coverage is 30.00%.

@@            Coverage Diff             @@
##           master     #204      +/-   ##
==========================================
- Coverage   67.48%   66.53%   -0.95%     
==========================================
  Files          20       20              
  Lines        3411     3419       +8     
==========================================
- Hits         2302     2275      -27     
- Misses        874      914      +40     
+ Partials      235      230       -5     

Impacted Files	Coverage Δ
output/json/json.go	29.16% <30.00%> (-2.09%)	⬇️
datasource/datasource.go	21.26% <0.00%> (-13.13%)	⬇️

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