Added Windows, MacOS and Linux builds

The 0.1-6 version marks the setup of automated system (GitLab CI + AppVeyor, if you're interested) that builds the Windows installer along with MacOS and Linux binaries of the game whenever I publish changes to the code. Now I can finally stop procrastinating and start fixing bugs and performance issues 😅

So this Saturday I'd be streaming a livecoding session where I'll be trying to fix some bugs, both on my Twitch and Youtube. Feel free to drop in and say Hi :)