Allow command-specific options in site aliases to work with sql-sync command

Imagine you want to skip comments in any sql-sync that involved a certain site alias, like so:

$aliases['gklive'] = array (
  'uri' => 'http://greenknowe.org',
  'root' => '/srv/www/drupal',
  'command-specific' => array (
    'sql-sync' => array (
      'skip-tables-list' => 'comments',
    ),
  ),
);

This would work just fine, except for the fact that the command has already been dispatched by the time the command-specific options are set, so skip-tables-list is never applied. This patch fixes this problem by immediately applying command-specific options as soon as they are set if a matching command has already been dispatched.