Added private Articulation._lilypond_format_bundle property.

All indicators will eventually receive a _lilypond_format_bundle property,
which LilyPondFormatManager will consult when formatting score components.
Implemented _storage_format_specification on LilyPondFormatBundle.

While not intended for persisting LilyPondFormatBundles to disk, being able to
visualize a bundle's state greatly helps with debugging.