Add Ronin::Exploits::SQLi#db_each_table and #db_each_column

Add a `db_each_table` / `db_each_column` methods to [Ronin::Exploits::SQLi](https://github.com/ronin-ruby/ronin-exploits/blob/master/lib/ronin/exploits/sqli.rb) which can [enumerate the tables/columns in the database](http://socketready.com/2011/06/23/basic-sql-injection/#names).
