Provide way to override/extend settings in pelican.auto.py

The pelican.auto.py file is generated from the template bin/pelican.auto.ezt with variables from pelicanconf.yaml.

This is somewhat inflexible - it is not currently possible for sites to add extra settings to the file, nor to change any fixed defaults.

It would be useful to be able to override/extend settings using the peliconconf.yaml file.