Add support for Telegram notifications

[TrAnn3l]

Hey, it would be great if you could add support for Telegram notifications.

You just have make a POST or GET request to specific URL with a few parameters.
The follwing docs show the basics:
https://core.telegram.org/bots/api#making-requests
And for the message one need to use the following method:
https://core.telegram.org/bots/api#sendmessage
The user would only need to supply the URL and the chat_id.
As message formatting, markdown and html is possible.

If you need any further information, feel free to ask.
Thank you in advance :)

[dholbach]

Would it makes sense to use https://github.com/containrrr/shoutrrr? It's used in weaveworks/kured now.

[stefanprodan]

@dholbach that library looks great 👍

[dholbach]

@atighineanu is in the #kured Slack channel and has helped out quite a bit.

[TrAnn3l]

Looks also great to me!

[somtochiama]

Picking this up