Use table prefix

Hi,

Brilliant plugin!
But I’ve noticed a problem when I was migrating a site with Duplicator:
your plugin creates a table in the database, but does not consider the table prefix set by wordpress (default is wp_).

Here is the fix:
edit line 15 in inc/class-wptg-db-table.php to
$this->table_name = $this->db->prefix."wptg_tables";