Allow runways to be specified

[timotheeg]

Most layouts support 2 runways as transition (start level + 1) and killscreen (29).

For beginner players, level 29 is too far off so the metric is not useful. Additionally for PAL, killscreen is 19, not 29.

It would be nice to allow runways boundaries to be explicitly set via an argument, such that someone might pass explicitly:

runways=start+1,19

or maybe even just

runways=19

and the default value would be:

runways=start+1,29