Minor unset_columns and display total changes

My first pull request :)

Added the option to pass multiple coma-separated columns to
unset_columns.
Modified display total to use count_all_results (mysql COUNT(*)) instead
of num_rows that gets all the records and the performs the count in PHP.

TODO: I'm considering the option to add custom functions instead of match-replacements to edit_column and add_column to build more complex row formats.