lilypond-book: filename option

[John Williams]

This resurrects the filename option and makes it do something useful.

If <lilypond filename="bach"> is given, "bach" will be used as the
basename for the files (instead of "lily-12345678").

This allows the files to have friendlier names, and for the snippets
to be managed better.

The previous filename option, which was rightfully deprecated, only made
a copy of the .ly file with the given name.  This option uses the
basename for all the extensions, including the .png file.

~ John Williams

filename.patch
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