Added CLI arg for site URL to EAD bulk export.

CLI tasks have no knowledge of hostname, etc., so can't automatically
generate a full base URL for exported XML. Added a CLI arg to the
EAD bulk export tool to specify a URL. Integrated this into the existing
EAD template.
Base URL now optional, change EAD identifier
The EAD identifier attribute in <eadid> now uses the top
level information object's slug instead of url_to().