nabc below staff

[davidweichiang]

This was brought up in #1559 but should probably be its own issue. Would this feature make #1626 unnecessary?

Similar to what was suggested in #1559, I can imagine

• nabc’s baseline goes \belowlinesnabcheight below the bottom line of the staff
• text’s baseline goes \spacelinestext below nabc baseline
• low notes cause nabc to be lowered according to \additionalbottomspacenabcthreshold

Much of the code is already set up for an arbitrary number of nabc voices. Does anyone ever want more than two? Or both voices above or both voices below?