Process missing modules in drush pm-list command.

Sometimes modules have been removed without being uninstalled. That leads to [serious performance problem](https://www.drupal.org/node/1081266). I suppose it would be handy to track such modules with  `drush pm-list` command.

![missing-modules](https://cloud.githubusercontent.com/assets/673139/4273054/82ab40f0-3ce7-11e4-9c95-31931114c396.png)