Adds DropTable builder

[belchior]

Adds DropTable builder

Builder to contruct a drop index command. Available only for the crate features postgresql and sqlite

Basic API

use sql_query_builder as sql;

let query = sql::DropTable::new()
  .drop_table("users")
  .as_string();

let expected = "DROP TABLE users";

assert_eq!(expected, query);

Output

DROP TABLE users