Normalize result path for each document

Hi!
I have a task to compile all posts to root folder. (I think many people stuck with that problem)
```
/src/posts/hello-world.html.md -> out/hello-world/index.html
```
Also I like to number my posts in source dir
```
/src/posts/001-hello-world.html.md -> out/hello-world/index.html
```
I started to write my own plugin, but then realize, that dateurls can solve all my requires (with cleanurls, of course).
All I need is to write in your new option
```
dateFormat: '/'
```
And just normalize path to avoid '//' in very start - this is what have I done in my fork.
I know it's some kind of unusual but witty way of using dateurls.