[v3.0.0] Check for supported ES2015 Features and update the code base

[michael-ciniawsky]

postcss-loader >= v3.0.0 drops support for node >= v4.0.0, therefore all ES2015 Features supported by node >= v6.0.0 can be used by postcss-loader >= v3.0.0

ℹ️ Development happens on the develop branch, please open PR's against this branch if you want to contribute :)

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