Add pm2 support

Add json file to launch and monitor in pm2 and allow automatic restarts
if the bot crashes

modmailbot-pm2.json

@@ -0,0 +1,8 @@
+{
+    "apps": [{
+        "name": "ModMailBot",
+        "cwd": "./",
+        "script": "npm",
+        "args": "start"
+    }]
+}