Support push notification via EU server

Thanks for the helpful tool.

Please update to offer CLI option to specify EU API (`api-eu.customer.io`) when sending push notification.
(I was able to download project and manually change the endpoint to get it to work, and don't have time to fork and offer patch atm)

Start at the CLI path that sends push notifications and locate where the API endpoint is selected. Check how existing CLI options are parsed, then verify that an EU option routes requests to api-eu.customer.io while the default behavior remains unchanged. The issue provides no test file, so identify the relevant command or diagnostic run for validation.

Written by the indexing model from the issue text.