feat: Add flag to override output destination

[thaney071]

Adds a flag to override the destination output.

The reason for adding a flag override in addition to the env setting is for easier usage with go generate and for automated scripting of readme generations since all other options are controled by flags.

This provides better access to using this tool for those not able to use Github actions.

Closes #100

[codecov[bot]]

Codecov Report

Merging #121 (6291301) into master (bbdf026) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #121   +/-   ##
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  Coverage   69.13%   69.13%           
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  Files           3        3           
  Lines          81       81           
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  Hits           56       56           
  Misses         15       15           
  Partials       10       10           

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[posener]

Hi! Note that the default behavior is to write the content to stdout. Maybe you can pipe stdout to a file of your choice? I would want to avoid adding too many flags.

Also - I suggest to split this to two PRs, one that adds the flag, and one that creates the parent directory in case it does not exist.