Fix Font-Face processing

[gverger]

Fixes #79

Proposed changes

This PR aims at fixing Font-Face recognition. As stated in #79 when the font family doesn't correspond exacly to its definition, Purgecss doesn't make the link between the two.

Types of changes

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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
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