Support for Lilypond for musical content

[mfhepp]

It would be good to add support for LilyPond, a widely used syntax for sheet music and other musical content.

The core is a CLI application that is available for all major platforms.

Here is a link to the essential command-line usage of LilyPond, and the full list of options and commands.

Lilypond can generate various output formats ( ps, pdf, png or svg) using the -f or --format=format option.

Reading input from stdin, which may be needed for pandoc-plot, works by using a dash (-) for the file argument.

Related issues and materials:

1. LilyPond filter in LUA
2. Minimal working example of Lilypond lua filter with markdown  pandoc/lua-filters#167

[LaurentRDC]

That's a great idea!

I don't have the time to implement this myself, but I am always happy to review pull requests