perf report: Add report.group config option

Add report.group config option for setting default value of event
group view.  It affects the report output only if perf.data contains
event group info.

A user can write .perfconfig file like below to enable group view by
default:

  $ cat ~/.perfconfig
  [report]
  group = true

And it can be disabled through command line:

  $ perf report --no-group

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/r/1358845787-1350-19-git-send-email-namhyung@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>