Add postinstall step

Author: swinton

package.json

@@ -5,7 +5,8 @@
   "main": "index.js",
   "scripts": {
     "test": "echo \"Error: no test specified\" && exit 1",
-    "lint": "eslint -f @microsoft/eslint-formatter-sarif -o output.sarif.json index.js"
+    "lint": "eslint -f @microsoft/eslint-formatter-sarif -o output.sarif.json index.js",
+    "postinstall": "cd eslint-formatter/src/ESLint.Formatter && npm install"
   },
   "repository": {
     "type": "git",