Ensure that the CLI output valid JSON

[Mayeu]

Proposed changes

This commit ensure that the CLI output valid JSON on stdout, so that it could then be manipulated in the shell using jq or other JSON manipulation tool.

Previously the output was not JSON valid making it hard to reuse.

I am also open to contribute an additional flag instead of making it the default.

Types of changes

What types of changes does your code introduce to Purgecss?
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• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

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• Lint and unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

[Ffloriel]

Thanks for the PR :)