Loop to insert or overwrite each label submitted by user
Only label columns if they actually exist in btable, otherwise raise error
Column labels now saved in persistence layer
Added parser for SHOW LABELS
Added engine function to return column labels
Pretty-print table for SHOW LABELS
Return data and columns for table instead of creating a new pretty-print style
Return column-label table for SHOW COLUMNS
Only print edited labels from LABEL COLUMNS
Added test for LABEL COLUMNS and SHOW LABELS
Add tablename to message after labeling columns