Adds support for postcss-cli style function config#14

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chrisdmacrae wants to merge 2 commits intostencil-community:masterfrom

chrisdmacrae:add-context-support

Commits on May 7, 2019

Upgrade postcss to ^7.0.16
chrisdmacrae
committed

Commits on May 9, 2019

Adds support for function return in postcss.config.js, matching postcss-cli
chrisdmacrae
committed