Complete the drush command for importing nodewords metatags

I'm in the process of upgrading a handful of old Drupal 6 site that were using nodewords, and I'm having trouble converting records. The drush command for converting from nodewords is commented out and code comments indicate that it doesn't work.

I've started to work on the drush command by copying how the page title drush command command works, and I've incorporated some of the working code from the nodewords UI tools. This first patch seems to do the trick for entities (node, user, term) and for the front page -- but hasn't been tested for anything else. Posting here incase it is valuable to others (and most likely my future self!)

TODO:
* Test / Add support for other nodewords types.
* Restore batching.
* Refactor so code is reused between UI and drush command