MusicXML: Implement pitch names for different languages

MusicXML: Implement pitch names for different languages

Added a --language (-l) command line switch for the musicxml2ly
script that allows you to specify the name of a language.ly file,
so that localized pitch names (e.g. do/re/mi/fa/sol/la/ti for
espanol) are used and \include "language.ly" is automatically
inserted into the lilypond file.

Unfortunately, I had to use a global variable in the musicxml.py
file, which holds the function to convert pitches to strings.
However, I found this much cleaner than passing the language or
the function through all functions calls...