Store args in JSON output

It would be useful to (optionally?) store the arg(s) in the output file.

Let's say you have a benchmark that has different performance for different sizes of N. And you want to view a plot the performance of that function across a relevant range of N. Currently, for both JSON and csv, the N value is stored embedded in the benchmark name. So in order to use N as an axis, you need to parse it out.

For JSON it would be nice to be able to import a dataset and use the Ns without parsing them out of the name field. This could be addressed at the same time as #814 - there could be a flag to switch between the current mode and a new mode of allowing multiple entries with the same "name" but different "args" entries. (I assume the reason that "name" and "run_name" are the same is in order to always have a unique "name" (?))

For plotting in, ie gnuplot, having a column(s) in the CSV with the args would also be useful. And maybe extra newlines between name blocks.