[2.x] Add Telegram notifications support

[snake03]

Added support for Telegram notifications using in your Envoy.blade.php:

@finished
    @telegram('<bot-id>','<chat-id>', '<message = null>', '<options = []>')
@endfinished

You should have a proper <bot-id>, the id of the bot you're using to send messages. You can get one using an existing Telegram Bot or creating a new one using BotFather.
Take a look at Telegram Docs for additional information about Bot ids.

You should also use a valid <chat-id>, which is the id of the chat you want to send notifications. You can use the id of your own Telegram account if you want to send messages to yourself through the Bot, or you can use the id of a Telegram group (in which the Bot is invited / present).
You can retrieve the id of your account or of a group in various way: a quick one is using @username_to_id_bot

The message and options params are optional.
Using message you can overwrite Envoy default message for a successful task.
Using options you can send additional params to Telegram Api.

Resolve Issue: #191

Sending to 2.x branch because I consider it a fully backward compatible minor feature.

Hope this can be useful. Thanks!