Added post-play-gcode feature

This change will make smoothie play a file after file has been played. It's meant to reset certain settings (e.g. set back to mm when the gcode-file was using inches, set back to absolute movement etc). It adds two config values:

post_play_gcode_enable                  true #enables post-playing a gcode file, default "false"
post_play_gcode                             /sd/gcode/reset.gcode #path to the gcode file default: "/sd/post_play.gcode -q"

I'm not sure how usefull this is to others, but it helped me understand smoothie better, and I found 2 bugs while coding it :)