perf tools: Add 'perf config' command

perf tools: Add 'perf config' command

The perf configuration file contains many variables to change various
aspects of each of its tools, including output, disk usage, etc.

But looking at the state of configuration is difficult and there's no
documentation about config variables except for the variables in
perfconfig.example exist.

So this patch adds a 'perf-config' command with a '--list' option.

    perf config [options]

    display current perf config variables.
    # perf config -l | --list

Signed-off-by: Taeung Song <treeze.taeung@gmail.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: Jiri Olsa <jolsa@redhat.com>
Link: http://lkml.kernel.org/r/1447768424-17327-1-git-send-email-treeze.taeung@gmail.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>