add a script for generating pngs of snippets
generate-pngs: setup lily build system if necessary
Add pngs to snippets:

input-shorthands/articulations-not-aligned-with-notes.ly
input-shorthands/late-evaluation-of-variables.ly
input-shorthands/voice-colors.ly
simple-examples/using-tie-configuration-property.ly
snippet-template.ly
specific-solutions/bracket-repeats.ly
templates/predefined-instruments/midi-channels.ly
templates/predefined-instruments/simple-example.ly