Named columns and null strings

Add two new on/off options that modify the generated COPY statement:

-w  issue a 'COPY' statement including column names to insert the data
-W  issue a 'COPY' statement without column names to insert the data (default)

-x  output empty strings in the input as NULL values in SQL
-X  output empty strings in the input as empty strings in SQL (default)

Default behavior is unchanged.