Track two more details about runs - the aggregate name, and run name. (#675)

This is related to @BaaMeow's work in https://github.com/google/benchmark/pull/616 but is not based on it.

Two new fields are tracked, and dumped into JSON:
* If the run is an aggregate, the aggregate's name is stored.
  It can be RMS, BigO, mean, median, stddev, or any custom stat name.
* The aggregate-name-less run name is additionally stored.
  I.e. not some name of the benchmark function, but the actual
  name, but without the 'aggregate name' suffix.

This way one can group/filter all the runs,
and filter by the particular aggregate type.

I *might* need this for further tooling improvement.
Or maybe not.
But this is certainly worthwhile for custom tooling.