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>

tools/perf/Documentation/perf-config.txt

@@ -8,6 +8,8 @@ perf-config - Get and set variables in a configuration file.
 SYNOPSIS
 --------
 [verse]
+'perf config' [<file-option>] [section.name ...]
+or
 'perf config' [<file-option>] -l | --list
 
 DESCRIPTION
@@ -118,6 +120,22 @@ Given a $HOME/.perfconfig like this:
 		children = true
 		group = true
 
+To query the record mode of call graph, do
+
+	% perf config call-graph.record-mode
+
+If you want to know multiple config key/value pairs, you can do like
+
+	% perf config report.queue-size call-graph.order report.children
+
+To query the config value of sort order of call graph in user config file (i.e. `~/.perfconfig`), do
+
+	% perf config --user call-graph.sort-order
+
+To query the config value of buildid directory in system config file (i.e. `$(sysconf)/perfconfig`), do
+
+	% perf config --system buildid.dir
+
 Variables
 ~~~~~~~~~