Create auto_bed_printing_area.pl

Post-processing script for the Roxy enhanced G29 gcode (http://3dprintboard.com/showthread.php?3105-Auto_Bed_Leveling-Enhanced-G29-command/)
This script replaces the custom Gcode parameters [G29XMIN], [G29YMIN], [G29XMAX], [G29YMAX] by their values.
You have first to add custom gcode in your slicing app like << G29 L[G29XMIN] F[G29YMIN] R[G29XMAX] B[G29YMAX] n4 >>