Add a SpawnHeightOffset configuration field.

Some pads (eg, the Hooligan Labs pad) have their origin at the bottom of
the pad and thus built craft spawn partially inside the pad, causing all
sorts of chaos. This change moves the built craft's start position up a
configurable amount (in meters). The recommended amount a little over the
distance from the pad's origin to its upper surface.

MODULE
{
	name = ExLaunchPad
	SpawnHeightOffset = 1
}