Add cs2 base image

[sazarkin]

Addressing this issue https://github.com/timche/docker-csgo/issues/39

This is minimal proposition to support current state of CS2 server.

We can add new tag for cs2, so older csgo tags are not affected.

Added new env variables: STEAM_USERNAME, STEAM_PASSWORD

You should create new steam account, add cs2 to library, and disactivate email steam guard.