Add lilypond package

[alerque]

This PR is to track progress on adding a package that will provide support for embedding Lilypond music in SILE documents.

• Merge Pass through Lua code in script #495 (changes to input grammar needed for this to work)
• Handle Lilypond file includes
• Handle inline Lilypond code
• Pass directory tracking info so Lilypond includes can be relative to the include file
• Add fragment mode to handle bare notes
• Pass information about current position in line so inline mode works as well as paragraph mode
• Make sure vertical space is available and reserved for each system before rendering (this will likely turn out to be a bug in the \img command that needs fixing first).
• Document usage

Will close #435.

[coveralls]

Coverage increased (+0.001%) to 95.51% when pulling 55a7b77 on alerque:lilypond into 26ceea6 on simoncozens:master.

[alerque]

I have another current use for this. Who was this maniac who didn't use conventional commits and didn't finish this up for merging?

[ctrlcctrlv]

Closed, unmerged changes should be made against https://github.com/sile-typesetter/lilypond.sile (see #435).