Adding ui.show-headers config file option to define if the histogram
entries headers will start visible or not.

Currently columns headers are displayed by default, following
lines in ~/.perfconfig file will disable that:

  [ui]
        show-headers = false
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Cc: Corey Ashford <cjashfor@linux.vnet.ibm.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1403886418-5556-4-git-send-email-jolsa@kernel.org
[ renamed symbol_conf.show_headers to .show_hist_headers ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>