Add ability to variable-delete (vdel) multiple variables at once

It would be nice to be able to delete multiple variables with a single command. I could see several ways to approach it:

For explicit lists:

1. Allow multiple command arguments, e.g., drush vdel --exact node_options_page node_preview_page node_submitted_page
2. Allow multiple picker selections, perhaps separated by spaces or commas.

For pattern matches:

1. Add an --all flag (or similar), e.g., drush vdel --all cron_
2. Add an "All" option to the picker after "Cancel".