Alpha22 is ready for you to test on Windows, Mac, and Linux. Besides adding support for all platforms, I've fixed a lot of bugs and issues that were reported by testers. Thank you all for helping out!
Alpha22 will be officially released soon, so this is the last chance to help with bug hunting. :)
You can opt into the testing branch by going into the game's properties in the Steam client, switching to the BETA tab, and choosing the branch 'dev'.
Please post any feedback in the comments below or on the community forums.