Ability to convert smallint to boolean

[beiske]

Of course this is something that can be handled in a separate step after the migration is done, but at least for my use case it would be smooth if I could specify a list of columns of type smallinteger that should be converted to boolean.

A more generic approach could be to create an easy way to extend the application with custom perl-methods for special case data-conversion.

[darold]

You now have the ability to convert on the fly any column value into boolean PostgreSQL value. In last commit you have two new configuration directives REPLACE_AS_BOOLEAN and BOOLEAN_VALUES to control this feature.

Call to custom Perl methods for special case data-conversion will be implemented in the future.