feat: add packageName option for docs.go

[hwwwi]

Describe the PR
swag is generating an invalid package name when the --output option is used with a hyphen.
Since using a hyphen in the package name is not allowed in go, replace it with an underscore.
Ex. --output my-swag will generate package my_swag

Also, I created a packageName option to specify the name of the package from generated docs.go file.
Ex. swag init --packageName my_swag will generate package my_swag.

Relation issue
fix #1441

[codecov]

Codecov Report

Patch coverage: 50.00% and project coverage change: -0.07 ⚠️

Comparison is base (aaa681e) 85.24% compared to head (973284b) 85.18%.

❗ Current head 973284b differs from pull request most recent head 607212e. Consider uploading reports for the commit 607212e to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1442      +/-   ##
==========================================
- Coverage   85.24%   85.18%   -0.07%     
==========================================
  Files          19       19              
  Lines        3654     3659       +5     
==========================================
+ Hits         3115     3117       +2     
- Misses        460      461       +1     
- Partials       79       81       +2     

Impacted Files	Coverage Δ
gen/gen.go	92.34% <50.00%> (-1.21%)	⬇️

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[ubogdan]

LGTM