Add completion subcommand

[nao1215]

This PR is for #28.

There are two problems with the automatic creation of shell completion files:

1. the creation of shell completion files without the user's permission may cause discomfort to the user
2. the deletion process is necessary because a shell completion file is created each time the root command is unit tested.

gup is designed to automatically create shell completion files to reduce user work. Therefore, the completion subcommand (default) of the cobra library is not used. Also, PowerShell support will be implemented as soon as possible.

@akimon658
Although you may not already use the original gup, I report to you.

[codecov]

Codecov Report

Merging #53 (2a6e6a4) into main (82477c7) will increase coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #53      +/-   ##
==========================================
+ Coverage   90.43%   90.49%   +0.06%     
==========================================
  Files          11       12       +1     
  Lines         460      463       +3     
==========================================
+ Hits          416      419       +3     
  Misses         28       28              
  Partials       16       16              

Impacted Files	Coverage Δ
cmd/root.go	100.00% <ø> (ø)
cmd/completion.go	100.00% <100.00%> (ø)

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[akimon658]

Awesome! I'll use the original gup again as it was not efficient to develop the fork.
Thank you for the update.