Usage
  
  * Default for a three column table :
  
  CREATE VIRTUAL TABLE pivot_table USING perl
      ("SQLite::VirtualTable::Pivot","base_table");
  
  * Specify the pivot columns:
  
  CREATE VIRTUAL TABLE pivot_table USING perl
       ("SQLite::VirtualTable::Pivot","base_table",
       "pivot_row", "pivot_column", "pivot_value");
  
  CREATE VIRTUAL TABLE pivot_table USING perl
       ("SQLite::VirtualTable::Pivot","base_table",
       "entity", "attribute", "value");
  
  * Can also transparently handle foreign keys in the base table:
  * <base table column>-><child table>.(<child table id>).<child table column>
  
   CREATE VIRTUAL TABLE pivot_table USING perl
        ("SQLite::VirtualTable::Pivot","base_table",
          "entity_id->entity_lookup(id).entity",
          "attribute_id->attribute_lookup(id).attribute",
          "value_id->value_lookup(id).value" );