Add YAML middleware that behaves like TOML and JSON

[Dekkonot]

After adding TOML in addition to JSON, it seems reasonable to also support YAML doing the same thing. However, I hate yaml, so I'm not sure whether we should have it be a default middleware.

I will leave it up to the committee (anyone who reads this) to weigh in on whether we should just by-default sync .yml files or if we should just expose a middleware for them.

[boatbomber]

I think that YAML is common enough that it should be supported out of the box, personal tastes aside. I don't think anyone likes YAML, but everyone uses it anyway.

[Kampfkarren]

I agree that yaml might make sense to just have on by default since it's obvious what the intent is if you put a .yml file in the codebase. No real passionate opinion there but I sway on default

Also are the docs stored in this repo? Should we update them?

[UpliftMacaw]

Docs would need to be updated here: https://github.com/rojo-rbx/rojo.space/blob/master/docs/sync-details.md