perf report: Add --stats option to display quick data statistics

Add --stats option to display quick data statistics of event numbers,
without any further processing, like the one at the end of the perf
report -D command.

  $ perf report --stat

  Aggregated stats:
             TOTAL events:       4566
              MMAP events:        113
              LOST events:         19
              COMM events:          3
              FORK events:        400
            SAMPLE events:       3315
             MMAP2 events:         32
    FINISHED_ROUND events:        681
        THREAD_MAP events:          1
           CPU_MAP events:          1
         TIME_CONV events:          1

I found this useful when hunting lost events for another change.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20180107160356.28203-12-jolsa@kernel.org
[ Rename it to --stats, plural ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>