add format tag func

[pei0804]

close #65

[codecov-io]

Codecov Report

Merging #102 into master will increase coverage by 0.18%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #102      +/-   ##
==========================================
+ Coverage   85.82%   86.01%   +0.18%     
==========================================
  Files           5        5              
  Lines         748      758      +10     
==========================================
+ Hits          642      652      +10     
  Misses         65       65              
  Partials       41       41

Impacted Files	Coverage Δ
operation.go	91.22% <100%> (+0.16%)	⬆️
parser.go	81.81% <100%> (+0.19%)	⬆️

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[easonlin404]

LGTM