Strip out index.html suffix from files that need it

We're thinking about using `jekyll-redirect-from` on https://guides.github.com/. The problem is that our directory structure is something like this:

```
── activities
│   ├── forking
│   │   └── index.md
│   ├── os-contributing
│   │   ├── index.md
│   └── socialize
│       ├── index.md
```

So, if I create a redirect like this:

```
redirect_from: /overviews/socialize
```

The redirect takes me to `activities/socialize/index.html`. I would prefer the clean URL of just `activities/socialize`.

Not sure at all how to test this, or if it's wanted. Let's discuss!