Feature request: add new env vaule for TELEGRAM API URL

[mikal]

According to notify page, "TELEGRAM_BOT_APITOKEN" and "TELEGRAM_BOT_CHATID" are required. Can the TG API URL, api.telegram.org, also be customized and then replaced with self-hosted API?

For example, https://${bot_API}/bot${bot_token}/getUpdates

In some scenarios, it would be useful to make TG work with the customized API. Thanks again for your great job.