Support postcss v8 (#349)

* Support postcss v8

Fixes #344

Closes #348, closes #345, closes #343.

* Don't promote global installs

README.md

@@ -16,7 +16,7 @@
 <h2 align="center">Install</h2>
 
 ```bash
-npm i -g|-D postcss-cli
+npm i -D postcss postcss-cli
 ```
 
 <h2 align="center">Usage</h2>

package.json

@@ -24,9 +24,8 @@
     "fs-extra": "^9.0.0",
     "get-stdin": "^8.0.0",
     "globby": "^11.0.0",
-    "postcss": "^7.0.0",
-    "postcss-load-config": "^2.0.0",
-    "postcss-reporter": "^6.0.0",
+    "postcss-load-config": "^2.1.1",
+    "postcss-reporter": "^7.0.0",
     "pretty-hrtime": "^1.0.3",
     "read-cache": "^1.0.0",
     "yargs": "^16.0.0"
@@ -37,11 +36,15 @@
     "eslint": "^7.8.0",
     "eslint-config-problems": "5.0.0",
     "nyc": "^15.0.0",
+    "postcss": "^8.0.4",
     "postcss-import": "^12.0.0",
     "prettier": "~2.1.0",
-    "sugarss": "^2.0.0",
+    "sugarss": "^3.0.0",
     "uuid": "^8.0.0"
   },
+  "peerDependencies": {
+    "postcss": "^8.0.0"
+  },
   "files": [
     "bin",
     "index.js",