Add a new configuration option: sentryDSN

Adding a new config option, ``sentryDSN`` to provide the Sentry DSN if
the user wants to monitor the BGPalerter app for runtime exceptions.

I have updated the package.json file to the version recommended in
https://docs.sentry.io/error-reporting/quickstart/?platform=node.

This is an optional config bit that enables Sentry monitoring only when
``sentryDSN`` is provided.

config.yml.example

@@ -176,3 +176,10 @@ checkForUpdatesAtBoot: true
 #      method: get
 
 pidFile: bgpalerter.pid
+
+############################
+#
+# sentryDSN: https://<key>@sentry.io/<project>
+#
+# Create a new project for monitoring BGPalerter on your Sentry server and grab
+# the generated DSN.