Add category to service list JSON

[mattstauffer]

(See #88 and #135)

[danielebarbaro]

I was thinking about json output days ago, feel free to assign also this to me 🤓. I can easily handle this feature. maybe we could talk about the right format

[mattstauffer]

@danielebarbaro Sounds great! We actually already have JSON output, so this would be a quick one for you :)

[mattstauffer]

https://github.com/tighten/takeout/pull/111