feat: ADD collection format flag

[wooongchi]

Describe the PR
Add collection format flag

ex) swag init --collectionFormat multi or swag init --cf multi

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.01 🎉

Comparison is base (9faf8b3) 85.19% compared to head (45d1713) 85.21%.

❗ Current head 45d1713 differs from pull request most recent head 7f89e6a. Consider uploading reports for the commit 7f89e6a to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1482      +/-   ##
==========================================
+ Coverage   85.19%   85.21%   +0.01%     
==========================================
  Files          19       19              
  Lines        3627     3631       +4     
==========================================
+ Hits         3090     3094       +4     
  Misses        459      459              
  Partials       78       78              

Impacted Files	Coverage Δ
gen/gen.go	93.54% <100.00%> (+0.02%)	⬆️
parser.go	94.52% <100.00%> (+0.01%)	⬆️

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