Ignore vendor folders to improve directory search performance

[SebCorbin]

See https://www.drupal.org/node/2329453

This is a critical issue for Windows users where drush does not ignore node_modules directory. A drush cc all will scan this directory and will find too long paths for Windows (due to node_modules nested tree).

To solve this for drush, an environment option to add some directories like node_modules, bower_components to the ignore directories passed in drush_scan_directory() would be really nice

[weitzman]

Looks like a dupe of #1347