Did you mean

Hi Zach,

This is a follow up on my previous pull request (https://github.com/holman/boom/pull/90), as my basic idea was not only to display a proper error but also a 'did you mean' thing à la git when you type something that does not exist. I wanted to add this as I never remember how I named my items.
So I played with the idea and implemented it. Feel free to refuse the pull request if you think this is an overkill as it adds another dependency.

Basically when you try to `copy` or `open` an item that does not exist you get this:

```
$ boom log
Couldn't find log.
Did you mean this?
    blog
```