Initial Implementation for display_segment update_method replace

[worldpeace-germany]

config_spec.yaml: changed default from not_set to off, left not_set in enum though I believe it can be removed. I couldn't see why not_set is a good default. If really needed for the changes in update_method replace I can add code to exchange not_set for off.

segment_display: Only changed code for update_method replace, stack should behave like before. I tested with various segment displays shows and it seems to behave well. Note:

• for replace the priority parameter is not used (and doesn't make sense if you don't have a stack)
• my shows work with replace, but completely misbehave if using stack
• I would propose (but have not done it since unsure about consequences for others) to change the default to replace in here https://github.com/missionpinball/mpf/blob/35a84ba29c54acc6ce1afab28bb0e9edca07e301/mpf/devices/segment_display/segment_display.py#L139
• I am not able to test color changes since my display is single color (though would believe it should work if it works for stack)
• flashing, transition, transition_out, expire parameter tested (and working)

If the PR gets accepted I would do some additons to mpf-docs 0.80. Just would need to know how to contribute version specific changes.