Still working but with errors

Although it shows son gui errors on the admin page, the plugin still works for 4.0. It would be nice to have them fixed.

Another nice improvement would be the ability of choosing the taxonomy type to use for mass delete. This can be easily tweaked by modifying plugin_mass_delete_unused_tags.php however an admin page option would be welcome.

For example,

$taxonomy = 'post_tag';

to

$taxonomy = 'product_tag';

for woocommerce tags.