More progress for issue #13 -- log management overhauled and multiple outputs prepared.
Nearing closure of issue #13 -- error feedback improved.

 * TeX engines report errors/warnings in a structured way.
 * Log message handling has been overhauled completely.
 * Log can be generated raw, as Markdown or as PDF (user option).
 * User can control the detail of the log.
 * CLI output indicates whether there have been errors.

Still to do: collect pandoc errors; overhaul extensions.