Support for nonacm option in acmart.cls

[dginev]

The nonacm option of acmart.cls disables the display of all "ACM reference format" metadata, which isn't yet implemented in LaTeXML.

To quote the original class file:

\AtBeginDocument{\@ACM@printacmreffalse}%
    % in 'nonacm' mode we disable the "ACM Reference Format"
    % printing by default, but this can be re-enabled by the
    % user using \settopmatter{printacmref=true}

We should have precise support, to avoid a misleading perception of the resulting metadata values (which often come from templates).

Reported by an arXiv author.