fix: moved to peer deps

Author: Mathieu Lavoie

package.json

@@ -43,7 +43,6 @@
   "dependencies": {
     "fancy-log": "^1.3.3",
     "plugin-error": "^1.0.1",
-    "postcss": "^8.0.0",
     "postcss-load-config": "^2.0.0",
     "vinyl-sourcemaps-apply": "^0.2.1"
   },
@@ -56,5 +55,8 @@
     "proxyquire": "^2.1.0",
     "sinon": "^6.3.5",
     "vinyl": "^2.2.0"
+  },
+  "peerDependencies": {
+    "postcss": "^8.0.0"
   }
 }