perf tools: Add gtk.<command> config option for launching GTK browser

Add config option for launching GTK browser for the specified command by
default.  Currently only 'report' command is supported.

Adding following line to the perfconfig file will have a same effect of
specifying --gtk option on command line (unless other related options
are not given).

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

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Acked-by: Pekka Enberg <penberg@kernel.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1352688617-25570-1-git-send-email-namhyung@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>