move {ARTICLE,PAGE}_DIR -> {ARTICLE,PAGE}_PATHS

Instead of one path a list can be given. This is due to popular request.
Should help people not wanting to use Pelican for blogging.
Maintain backward compatibility though.
Thanks to @ingwinlu for pointing out the change in StaticGenerator.
Fix #1344 move PLUGIN_PATH -> PLUGIN_PATHS

Pelican uses *_PATHS names for settings that represent a list of paths.
Add {PAGE,ARTICLE}_PATHS to {ARTICLE,PAGE}_EXCLUDES automatically

This makes it easier for someone to change PAGE_PATHS without the
need to change ARTICLE_EXCLUDES accordingly.