Implement file level attributes to control behavior for single sql scripts

 - Initial use case is to enable control of the --single-transaction parameter used
   by default when applying a sql script
 - This change introduces attributes on each file that appear in comments. For example,
   in a migration script, adding:

   -- attribute.transaction=single

  reflects the current behavior (and remains the default)

  -- attribute.transaction=none

  will remove the "--single-transaction" flag from the psql commant