Also delete pairs on backward-delete-word

[goonzoid]

This is sort of a fix for #18, although in my shell, Alt + Backspace doesn't seem to delete a whole word. Ctrl + w does, though (it's a readline binding, if I'm not mistaken) and this makes it work in the same way that single character deletion does.

It's not impossible that I missed some edge cases, so let me know if you think that might be the case and I can dig a little deeper.

Thanks for a great plugin!