perf config: Add support for getting config key-value pairs

Add a functionality getting specific config key-value pairs.
For the syntax examples,

    perf config [<file-option>] [section.name ...]

e.g. To query config items 'report.queue-size' and 'report.children', do

    # perf config report.queue-size report.children

Signed-off-by: Taeung Song <treeze.taeung@gmail.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Nambong Ha <over3025@gmail.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Wang Nan <wangnan0@huawei.com>
Cc: Wookje Kwon <aweee0@gmail.com>
Link: http://lkml.kernel.org/r/1478241862-31230-2-git-send-email-treeze.taeung@gmail.com


[ Combined patch with docs update with this one ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>