Allow multiple chat_id for Telegram storage.

[Gurulhu]

It may be a corner case and I understand this can be "solved" by putting the bot in a shared chat, but I bumped into the issue of wanting to notify more than one user on Telegram, and as it is now the implementation only supports one chat id.

As a side note, the telegramstorage.py file is missing a PastieStorage import that sometimes silently breaks the storage class if no other storage class that imports it is enabled and loaded before it.

I fixed both issues in my fork and will open a PR if you deem the multiple chat_ids issue relevant.
If you chose to deny it, I can rollback the fork and PR only to fix the import issue, just let me know.

[cvandeplas]

Such contribution would be nice. Thank you