nitcorn: add utility to produce HTTP error pages (and use it in FileServer)

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
nitcorn: custom header in FileServer

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
contrib: intro the benitlux project

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
examples: add the config of xymus.net as a nitcorn example

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
pep8analysis: use header of xymus.net

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>