Realtime logging for latexmlc

I wanted to keep the default realtime logging behaviour of latexml (printing to STDERR as the conversion goes along) when latexmlc isn't communicating to a server.

I added that feature in latexmlc, also adding a convenience method "push" for LaTeXML::Util::Config options.

That revealed a regression in LaTeXML::Util::Config in actually activating the DEBUG variables in each requested LaTeXML class.

It also revealed a timing issue with the activation in LaTeXML::Converter, which does bind_log pretty early on (and the DEBUG flag disables the binding/flushing of a log variable. That's ALL it does for the moment, and I think all it should ever do). 

With all these in place, latexmlc now behaves exactly like latexml unless instructed otherwise (via an --expire or --address option).