Intertext in display mathematics

[Rob Beezer]

I have made some very significant changes to accomodate our #intertext element
inside display mathematics (#md). Mostly this will

* address a minor bug in the EPUB conversion
* help with formats using "offline math" (via offline MathJax), such as braille
* preserve the \intertext{} feature in LaTeX, preserving alignments

You should not notice any changes in your output (it is now just a bit better),
nor should you need to change any of your source. So why announce here?

Holler if you like this feature and you notice bugs.

But mostly to emphasize that, other than LaTeX/PDF output, no electronic format
is going to be able to preserve alignments across the "intertext" elements.
Just too dang hard, I guess. It'd likely be MathJax that does it, and they
don't. But when they do, we will be ready. For LaTeX/PDF you will still get
alignment across the intertext, so your print materials will still have
high-quality typesetting.

Read "pretext-dev" if you want all the technical details.

Rob