nsf2wav: Fix mono, add flag options, and more.

[eatnumber1]

This change is a decently large clean up / improvement to the nsf2wav tool, and includes:

• Add options and comprehensive help text to nsf2wav.
• Make the samplerate configurable via a flag option.
• Fix rendering to mono, which previously required changing a macro, but didn't work correctly even when that change was made. It is now also made available as a flag option.
• Allow specifying track length and fade out time (only used for tracks which don't already specify their times).
• Move the track number from a mandatory argument to a flag option.
• Readability improvements in the code.

It has also now been tested and works correctly on macOS Catalina 10.15.

[eatnumber1]

@jprjr in case you want to take a look.