Add regex for Telegram Bot API keys

[PurkkaKoodari]

Telegram provides its Bot API for users to create chatbots. I've seen the amount of bots grow a lot since the API was released, and I'm pretty sure Pastebin has a lot of their API keys. (I have accidentally posted some myself, not in public pastes though.) So, here's a patch to add matching for them.

The regex used is \d{9}:[0-9A-Za-z_-]{35}, which shouldn't produce many false positives, and matches all API keys I have seen.