Add postcss-clean dep for minification

Re #90

Author: JayPanoz

css/postcss.config.js

@@ -234,6 +234,10 @@ module.exports = (ctx) => ({
       ],
       "unspecified-properties-position": "bottomAlphabetical"
     }),
+    require("postcss-clean")({
+      advanced: false,
+      roundingPrecision: 3
+    }),
     require("postcss-header")({
       header: `/*\n * Readium CSS (v. ${version})\n * Developers: Jiminy Panoz \n * Copyright (c) 2017. Readium Foundation. All rights reserved.\n * Use of this source code is governed by a BSD-style license which is detailed in the\n * LICENSE file present in the project repository where this source code is maintained.\n*/`
     })

package-lock.json

@@ -28,6 +28,7 @@
     "backstopjs": "^4.5.0",
     "express": "^4.17.1",
     "postcss-alter-property-value": "^1.1.3",
+    "postcss-clean": "^1.1.0",
     "postcss-cli": "^7.1.0",
     "postcss-css-variables": "^0.14.0",
     "postcss-custom-media": "^7.0.8",