Consider making countIncrementalSlides: false the default

Currently this can be done by tweaking `*.body.html` with:

```js
var slideshow = remark.create({
   // Enable or disable counting of incremental slides in the slide counting
  countIncrementalSlides: false,
  });
```

But this option might be useful for everybody. See also https://github.com/gnab/remark/wiki/Configuration.