*Display* aggregates only. (#665)

There is a flag 
https://github.com/google/benchmark/blob/d9cab612e40017af10bddaa5b60c7067032a9e1c/src/benchmark.cc#L75-L78
and a call
https://github.com/google/benchmark/blob/d9cab612e40017af10bddaa5b60c7067032a9e1c/include/benchmark/benchmark.h#L837-L840
But that affects everything, every reporter, destination:
https://github.com/google/benchmark/blob/d9cab612e40017af10bddaa5b60c7067032a9e1c/src/benchmark.cc#L316


It would be quite useful to have an ability to be more picky.


More specifically, i would like to be able to only see the aggregates in the on-screen output,
but for the file output to still contain everything. The former is useful in case of a lot of repetition
(or even more so if every iteration is reported separately), while the former is **great** for tooling.

Fixes https://github.com/google/benchmark/issues/664