Good way to collaborate on Fountain files?

I'm writing a script in Fountain. I usually write alone, but this time I'm collaborating with a co-writer. What's a good way for us to work on the script together?

We tried a Google Doc, but it's missing so many good screenwriting features like autocomplete that it's kinda frustrating.

I'm aware of WriterDuet. It sounds pretty good, but is there another option that's free? One of my favorite things about Fountain is how versatile it is; I'm hoping that that versatility leads to a good free collaborative solution.

Thanks!