So pretty much the same thing we had before, except instead of writing JSON you write YAML
Example: https://github.com/Benvie/JavaScriptNext.tmLanguage/blob/mas...
Example: https://github.com/Nessphoro/sublimeassembly/blob/master/Ass...
Output: https://github.com/Nessphoro/sublimeassembly/blob/master/Ass...
I suspect you're confusing what some juiced-up workflow makes possible vs. how stock Sublime works.