fix: respect the $PORT environment variable

Author: billiegoose

package.json

@@ -4,7 +4,7 @@
   "description": "When you need a file, but the headers ain't good, who you gonna call? CORS Buster!",
   "main": "index.js",
   "scripts": {
-    "start": "micro",
+    "start": "cross-env micro --listen=tcp://0.0.0.0:$PORT",
     "dev": "micro-dev"
   },
   "keywords": [],
@@ -15,6 +15,7 @@
     "url": "git://github.com/wmhilton/cors-buster.git"
   },
   "dependencies": {
+    "cross-env": "^5.1.4",
     "micro": "^9.1.4",
     "node-fetch": "^2.1.2"
   },