PreTeXt "internal" namespace

[Rob Beezer]

A heads-up mostly, as you contribute to replying to support questions (which is
really, really, really appreciated!).

We often make up ad-hoc XML markup for auxillary files and other uses, and have
mostly been careful about putting these elements and attributes into a "pretext
internal" namespace, using a "pi" prefix/alias. But we don't really want
authors using these elements.

For example, most recently, we have been converting references (bibliography
items) from semantic markup to a more presentational markup for subsequent
(easy) tweaking into final output forms. In the extreme, we have a #pi:bold
element! This would enable an author to create the mother of all end-runs
around the PreTeXt philosopy. Fortunately, we have a pre-processor.

So now any *authored* element or attribute in the pretext internal namespace
will get killed very early in pre-processing. Not expecting problems, but
consider this change a culprit if we get some reports of serious or bizarre bugs.

Thanks,
Rob