Add helpers for t.me/ links

In #854, I suggested to also add extract_url helpers for `t.me/..` links:

![image](https://user-images.githubusercontent.com/7313176/73142068-e16e7b00-408a-11ea-97c9-3e93875f5d6e.png)

https://github.com/python-telegram-bot/python-telegram-bot/pull/854#issuecomment-566120162

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