Accept variables number of arguments in drush_shell_cd_and_exec().

Impossible to pass dynamically builded commands to the ```drush_shell_cd_and_exec()```

Does not work:
``` php
$wd = '/foo';
$cmd = 'mycommand %s %s';
$args = array('foo', 'bar');
drush_shell_cd_and_exec($wd, $cmd, $args);
```

This does not works, but with the patch:
``` php
$wd = '/foo';
$cmd = array(
  'mycommand %s %s',
  'foo',
  'bar',
);
drush_shell_cd_and_exec($wd, $cmd);
```