Integrate validation stylesheet into Python routines

[rbeezer]

The "validation plus" stylesheet could be integrated into the core routines backing the pretext/pretext script.

Along with output from Jing, each line could be busted up into components, then sorted/presented in different ways.

In particular:

• Each error message exactly once.
• All messages, sorted/grouped by error message text.
• All messages, in document order (the default).

[kcrisman]

Great idea. Would it be accessible in the CLI? That could help induce this author to make the switch.

[rbeezer]

Eventually. We need to get jing to come along first. I have ideas.