feat: support json output

[G-Rath]

This originally started as a general output re-design, but turned into supporting json output.

I'm somewhat annoyed at this because outputting in the loop is nice but when you're outputting JSON you want to do it as an array and... ugh it just kind of breaks the single responsibility principle.

However I think I've found a good solution: having a Reporter class that tracks this (and so owns the responsibility for outputting) - all that functions are required to do is produce "results" which contractually must be convertible to a "string" format and to json (which is implied).

The main icky thing with this is that you need to ensure that #Finish is called, which is technically an implication detail but afaik there's no escaping it - really, this is a form of RAII that's made icky by the fact that you have to make it that by setting up the defer (e.g. it'd be better if the class itself could somehow setup the defer).

I've made this a bit nicer by renaming Finish to PrintJSON which then means its the responsibility of the "user" to know when/if they want to call that.

Anyway, this should be fine I think - hopefully I've not missed something that makes it horrible to use.

Resolves #10