Add an option to enable notifications (-N, --notify) and disable them by default.

[nao1215]

I will make a correction for Issue #56.
I would like to add an option to enable notifications (-N, --notify) and disable them by default.

[codecov]

Codecov Report

Merging #57 (89d5aa6) into main (984a90e) will increase coverage by 0.71%.
The diff coverage is 95.45%.

@@            Coverage Diff             @@
##             main      #57      +/-   ##
==========================================
+ Coverage   90.49%   91.21%   +0.71%     
==========================================
  Files          12       12              
  Lines         463      478      +15     
==========================================
+ Hits          419      436      +17     
+ Misses         28       27       -1     
+ Partials       16       15       -1     

Impacted Files	Coverage Δ
cmd/update.go	72.16% <92.30%> (+4.38%)	⬆️
cmd/check.go	96.49% <100.00%> (+0.19%)	⬆️
cmd/import.go	90.32% <100.00%> (+1.86%)	⬆️

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