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 see what you mean about the lyrics, and I can see a rationale for not scaling padding.  That might be best, but then I wonder about elements that only have padding and no basic-distance or minimum-distance -- then they won't have their spacing scaled at all (top-markup-spacing, chord-names, figured-bass, note-names-nonstaff-unrelatedstaff).  So I probably need to think and work more on this.  

Huh, I just tried the example with a modified version of the function that doesn't touch padding and the lyrics spacing is the same...