[FR] Allow to add more information in the custom output for json

[DenisYaroshevskiy]

I generate benchmarks with different variations.
However, last I checked, the only field I can populate is benchmark_name.
So I first populate the name with the things I need:
https://github.com/DenisYaroshevskiy/unsq_eve/blob/a2c914f0c37e57911ba47751db0463a33f48bf22/src/bench/bench.h#L100
And then parse them out in the visualization.

It would be great to have some way to add an object to the outputted json.

[dmah42]

anything passed as an Arg should be included already in the JSON and ArgName can be used to override the value that would otherwise be included.

however this sounds like a really good idea. :)