Upgrade to purgecss v7

Author: markdumay

config/postcss.config.js

@@ -4,7 +4,9 @@ const cssnano = require('cssnano')({
   preset: 'advanced'
 })
 const whitelister = require('purgecss-whitelister')
-const purgecss = require('@fullhuman/postcss-purgecss')({
+const purgeImport = require('@fullhuman/postcss-purgecss')
+const purgeCSSPlugin = purgeImport.purgeCSSPlugin || purgeImport.default || purgeImport
+const purgecss = purgeCSSPlugin({
   content: ['./hugo_stats.json'],
   defaultExtractor: (content) => {
     const els = JSON.parse(content).htmlElements