Add user-init option to include a custom init snippet (not replacing the whole init file)

scm/lily.scm

 ;;;; This file is part of LilyPond, the GNU music typesetter.
 ;;;;
 ;;;; Copyright (C) 1998--2010 Jan Nieuwenhuizen <janneke@gnu.org>
 ;;;; Han-Wen Nienhuys <hanwen@xs4all.nl>
 ;;;;
 ;;;; LilyPond is free software: you can redistribute it and/or modify
 ;;;; it under the terms of the GNU General Public License as published by
 ;;;; the Free Software Foundation, either version 3 of the License, or
 ;;;; (at your option) any later version.
 ;;;;
@@ skipping 151 matching lines @@
 "Don't use directories from input files while
 constructing output file names.")
     (svg-woff #f
 "Use woff font files in SVG backend.")
     (trace-memory-frequency #f
 "Record Scheme cell usage this many times per
 second.  Dump results to `FILE.stacks' and
 `FILE.graph'.")
     (trace-scheme-coverage #f
 "Record coverage of Scheme files in `FILE.cov'.")
+    (user-init #f
+"Global settings file, included before the score is processed.")
     (verbose ,(ly:command-line-verbose?)
 "Value of the --verbose flag (read-only).")
     (warning-as-error #f
 "Change all warning and programming_error
 messages into errors.")
     ))
 
 ;; Need to do this in the beginning.  Other parts of the Scheme
 ;; initialization depend on these options.
 
@@ skipping 659 matching lines @@
             (ly:exit 0 #f)))))
 
 (define (gui-no-files-handler)
   (let* ((ly (string-append (ly:effective-prefix) "/ly/"))
          ;; FIXME: soft-code, localize
          (welcome-ly (string-append ly "Welcome_to_LilyPond.ly"))
          (cmd (get-editor-command welcome-ly 0 0 0)))
     (ly:message (_ "Invoking `~a'...\n") cmd)
     (system cmd)
     (ly:exit 1 #f)))