Add flag to generate documentation only for specifics tags

[Trial97]

Describe the PR
Added CLI flag to generate the doc for them if specified.
It helps when you want to generate multiple pieces of documentation(e.g. one for devs and a separate one for customers)

Relation issue
None

Additional context
ToDo:

• Add a flag to the parser
• Add tests

[ubogdan]

@Trial97 Not sure about it.

[Trial97]

@ubogdan Feel free to close it if this is not desired :D.
But why not? Does this PR lack in some way or form?

[ubogdan]

@Trial97 I'll like to hear @sdghchj opinion.

[codecov]

Codecov Report

Base: 95.71% // Head: 95.76% // Increases project coverage by +0.04% 🎉

Coverage data is based on head (5d3f2e0) compared to base (eaed517).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1379      +/-   ##
==========================================
+ Coverage   95.71%   95.76%   +0.04%     
==========================================
  Files          14       14              
  Lines        2826     2855      +29     
==========================================
+ Hits         2705     2734      +29     
  Misses         66       66              
  Partials       55       55              

Impacted Files	Coverage Δ
gen/gen.go	93.45% <100.00%> (+0.03%)	⬆️
parser.go	93.90% <100.00%> (+0.21%)	⬆️

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[sdghchj]

@ubogdan
I agree on this feature, for my team had also asked for it some days ago.
We usually need a piece of open API for customers and another piece for develpers in our own team.

[ubogdan]

LGTM