Proxy support and markdown escaping

[ertaquo]

trafficstars

Added proxy support. Reworked notify method, don't know why. Just seen it in other plugins. Escaped markdown characters. That's important, because if you have project name with underscore character (like "backend_dev"), it will break Markdown syntax, and Telegram will return error. Used markdown-strings==2.1.3, because newer versions doesn't work properly.

[gituser]

Hey @butorov can you merge this?

This is useful when you have certain characters in the title of the issue or inside the message.

Proxy support not sure, but escaping is needed most definitely.