Use the font-family library to parse font-family property in postcss ast

[papandreou]

Prevents the backslash from being kept in font-family: Font Awesome\ 5 Free

Fixes #128

[coveralls]

Pull Request Test Coverage Report for Build 586

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.2%) to 90.416%

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Files with Coverage Reduction	New Missed Lines	%
lib/HeadlessBrowser.js	2	91.43%
<!--	Total:	2		-->

Totals
Change from base Build 583:	-0.2%
Covered Lines:	1154
Relevant Lines:	1227

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💛 - Coveralls