fix: Allow Absolute Paths

[Subash]

Currently PurgeCSS only allows relative config path.
For example,

purgecss --config  abc/purgecss.config.js

This will allow absolute paths such as

purgecss --config /Users/subash/Desktop/Test/purgecss.config.js

Proposed changes

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• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
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• Lint and unit tests pass locally with my changes
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[Ffloriel]

Thanks!