add default to produce output label file if no output flags

[nvkelso]

Seems that if only a file name is provided, it would produce this:

-p PLACES_FILE, --places-file=PLACES_FILE

That way something useful would be generated instead of slapping the user's hands.

If more than one file is specified, then enforce the flags for type and filename:

-l LABELS_FILE, --labels-file=LABELS_FILE
Optional name of labels file to generate.
-p PLACES_FILE, --places-file=PLACES_FILE
Optional name of place points file to generate.
-r REGISTRATIONS_FILE, --registrations-file=REGISTRATIONS_FILE
Optional name of registration points file to generate.
This file will have an additional "justified" property
with values "left", "center", or "right".