perf stat: Add support to print counts for fixed times

Introduce a new option to print counts for fixed number of times and
update 'perf stat' documentation accordingly.

Show below is the output of the new option for perf stat.

  $ perf stat -I 1000 --interval-count 2 -e cycles -a
  #           time             counts unit events
           1.002827089         93,884,870      cycles
           2.004231506         56,573,446      cycles

We can just print the counts for several times with this newly
introduced option. The usage of it is a little like 'vmstat', and it
should be used together with "-I" option.

  $ vmstat -n 1 2
  procs ---------memory-------------- --swap- ----io-- -system-- ------cpu---
   r  b swpd   free   buff   cache    si   so  bi   bo  in   cs us sy id wa st
   0  0    0 78270544 547484 51732076  0   0   0   20    1    1  1  0 99  0 0
   0  0    0 78270512 547484 51732080  0   0   0   16  477 1555  0  0 100 0 0

Changes since v3:
- merge interval_count check and times check to one line.
- fix the wrong indent in stat.h
- use stat_config.times instead of 'times' in cmd_stat function.

Changes since v2:
- none.

Changes since v1:
- change the name of the new option "times-print" to "interval-count".
- keep the new option interval specifically.

Signed-off-by: yuzhoujian <yuzhoujian@didichuxing.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Kan Liang <kan.liang@intel.com>
Cc: Milian Wolff <milian.wolff@kdab.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/r/1517217923-8302-2-git-send-email-ufo19890607@gmail.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>