Add support for %%writefields

[seisiuneer]

Jean-François Moine on the abcusers group mentioned:

"Some old parameters/commands as %%printtempo, %%withxrefs, %%musiconly and %%writehistory are not needed anymore with %%writefields."

So I thought it best to at least memorialize the idea.

It's above my understanding of the abcjs internals to do myself.