Add "Scale vertical spacing"

Provides an easier way to adjust vertical spacing, letting you scale the default settings, see:
http://lilypond.org/doc/v2.18/Documentation/notation/flexible-vertical-spacing-paper-variables
http://lilypond.org/doc/v2.18/Documentation/notation/flexible-vertical-spacing-within-systems
I think this should be removed, as we already have `example.ly`?