tools/power turbostat: add --out option for saving output in a file

By default...

Turbostat --debug gconfiguration info goes to stderr.

In FORK mode, turbostat statistics go to stderr.

In PERIODIC mode, turbostat statistics go to stdout.

These defaults do not change, but an option "--out file"
will send all output above only to the specified file.

Signed-off-by: Len Brown <len.brown@intel.com>