Added JSON output option

[grahambrooks]

Reflects the cloc output format but does not output a "header" object.

I needed a faster lines of code program for some historical analysis and came across loc. Certainly much faster but I needed an easily parsable output format - JSON.

Hope the updates are useful.

[cgag]

I want to experiment with serde a bit for my own learning, but I have to admit I forgot how simple just doing string interpolation could be, I may end up going with this instead.

Thanks a bunch for the PR, sorry for the huge delay in replying.