Enhancement: Add recursive option

[nhi-vanye]

I have a deeply nested directory of archived JSON responses that I would like to analyze (extract fields from each of the response files)

find -exec gron would probably work, but the hierarchy holds important context so I'd want to have the file path included in the output.

I'm thinking something like grep -r

[RossPatterson]

I needed the same thing, but rather than implement grep -r, I did grep -H, so find -exec gron -H gets you the filename. See https://github.com/RossPatterson/gron (or #72)

[RossPatterson]

@tomnomnom Can you review my pull request #72 for this feature?