Feature Request: folding by directory

Wouldn't it be awesome if bufexplorer had a possibility to expand/contract directories?

Something like

```
a/a/a.txt
a/a/b.txt
a/a/c.txt
a/b/a.txt
a/b/b.txt
a/b/c.txt
```

to

```
<a/a/...>
<a/b/...>
```

or

```
<a/...>
```

using vim folding e.g.
