Add $PUBLICATION_TYPE config setting, and add validation of config.

$PUBLICATION_TYPE doesn't currently do anything, and defaults
to 'numbered'.
Add 'dated' publication type.

In the config file you can set `$PUBLICATION_TYPE` to `dated`
and create edition files with specific dates. Rather than have
a sequential series of numbered editions.

Updated README with instructions.
Add LP-PHP-Miniseries HTTP Header