adding additional configurable options

- default site author, override-able on a per-post basis
- made the config separator in the post format configurable on a per-site basis
- changed the file migrations to use rsync rather than cp.
- make the build check in the local directory for a config file before falling back to the user-wide config file.
- made "posts" directory configurable 
- made output extension configurable, including theoretical support for having extension-less paths (though this requires some corresponding web server configuration)