Added functionality for specifying output file for KeyLogger plugin

Hi Dominic,

I have created a small patch for allowing a commandline option (-K) on usb-mitm for specifying another output file than stderr on the KeyLogger plugin. I tried to keep all codestyle/tabs etc the same.
On my box, I make a fifo: mkfifo /tmp/key_output
and start usb-mitm with -p 1200 -K /tmp/key_output
and another process cat /tmp/key_output
and this seems to work as expected. I also added a fflush to the KeyLogger output because else it is not flushed.
If you have any objections, please let me know.
Kind regards,

Jaap Crezee