Added onExit handler to masterPlugins (see description)

To use the onExit handler, simply add a method called onExit to your exported class. If it returns a promise, the master will wait for it to resolve.

package.json

@@ -30,8 +30,8 @@
     "ejs": "^2.5.7",
     "express": "^4.16.2",
     "express-fileupload": "0.0.6",
-    "follow-redirects": "^1.4.1",
-    "fs-extra": "^1.0.0",
+    "follow-redirects": "^1.5.0",
+    "fs-extra": "^6.0.1",
     "getmac": "^1.4.0",
     "hash-files": "^1.1.1",
     "html5-validator": "^1.0.7",