Add posibility to drop table IF EXISTS and CASCADE

[Tigrov]

Add two options for dropping a table

1. IF EXISTS - drop table only if it exists
2. CASCADE - drop also dependencies cascade

Suggestions for QueryBuilderInterface::dropTable()

• Change argument string $table to array|string $tables and realize method for list of tables;
• Add second argument $cascade = false and generate SQL query to drop tables and dependencies cascade;
• Add new method QueryBuilderInterface::dropTableIfExists() with the same agruments and generate SQL query with IF EXISTS construction.