adds support for null suppression

This is controlled via the "\pset suppress_nulls=on" command.
Rows with all null values will be suppressed.
Columns with all null values will be suppressed.
Suppression only applies to formatted (tabular) output, not to CSV output.
When null-suppression mode is on, only one RowBuffer worth of (non-suppressed)
rows will be fetched; this limit is currently hard-coded to 50 rows.