Fix sql-sync not propagating options to sub-commands

Currently, the following example at `examples/example.aliases.drushrc.php` does not work:
    
    ```
     #$aliases['live'] = array(
     #    'parent' => '@server,@dev',
     #    'uri' => 'mydrupalsite.com',
     #     'target-command-specific' => array (
     #       'sql-sync' => array (
     #         'skip-tables-list' => 'comments',
     #       ),
     #     ),
     #  );
    ```
 The reason is that skip-tables-list is an option for sql-dump, not for sql-sync. The above would only work if the setting was set to the sql-dump command instead of the sql-sync command. However, this may be desired just for sql-sync so it is not a good solution. Besides, this must be frustrating developers which do not understand why tables are not ignored.

We should propagate sql-sync options to its sub-commands such as sql-dump or sql-sanitize. Perhaps on a follow up we could remove options which belong to sub-commads defined at sql-sync and instead use `allow-additional-options => ('sql-create', 'sql-dump', 'sql-sanitize')`.