Implement proper/better IPC between plugin and xmobar

Implement named-pipe with proper permission of 0600 for Inter-Process
Communication between Xmobar and this plugin. Update README.md
accordingly.

ADVISORY: other system users could before write into ~/.pidginx and
deface your status bar or worse..

Signed-off-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>