add --pp-json - --json with pretty print

[yarikoptic]

it is frequent desire to use/see produced .json by a human. ATM it requires additional piping through jq or alike.

I think it might be beneficial, especially for use with --outfile to have --pp-json mode.

But I am about to suggest one more mode ( ;) ) so I wonder if it should altogether RFed into -f|--output-format text| json| json_pp to

• facilitate possible expansion if needed
• might be cleaner in the code internally ;-)