perf tools: Add --debug optionto set debug variable

Adding --debug option as a way to setup debug variables.  Starting with
support for verbose, more will come.

It's possible to use it now with report command:
  $ perf --debug verbose   ...
  $ perf --debug verbose=2 ...

I'll need this support to add separated debug variable for ordered
events change in order to separate debug output out of standard verbose
stream.

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: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@gmail.com>
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/20140717105500.GG516@krava.redhat.com


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>