Fix #672

Fix #672

Reported by Renaud Flavigny.

The output directory name sometimes contain spaces or other characters
that should be escaped, but lilypond-book does not escape characters
neither quote the snippets file name, which makes lilypond crash.

The bug can be reproduced by writing an input file test.tex in a
directory path that contains spaces and running "lilypond-book
--output=out test.tex".