Please add support for M98 and M99

[ospkant]

Please add support for M98 and M99. Currently, gcode.h has M99, but I don't know what it completes.

[terjeio]

M98 is somewhat difficult to implement - the gcode has to be preparsed before it is run and is has to be run from SD card since the file pointer has to be repositioned when the subroutine is called and when it exits. So not on my todo list for now.

Use G65 or O-word macros/subroutines instead, M99 is return from macro/subroutine. Note that macros/subroutines can only be run from SD card (local file).