add custom starting point of foreground horsehoe

[HighspeedTrain1]

-added option "start" to "horseshoe_scale"
-if start is not set it will default to the current functionality(starting at the left most point)
-two horseshoes are rendered at the starting point specified(one regular and one mirrored to go in the other direction for values < start)

Note that this will NOT always start in the middle but rather at the starting point value

Examples: (horseshoe value = top number)

horseshoe_scale:
  min: -100
  max: 100
  start: 0

horseshoe_scale:
  min: -100
  max: 100
  start: 50

[Ndrinta]

Ha! We had the "same" idea :) have a look -> #113