Added a modifier mechanism for rounding interpolated values

I added a simple mechanism for (optionally) applying modifications to interpolated values. I needed this for animating the `src` property of `<img>` elements. Imagine this:

```html
<img src="image_0.jpg" data-top="@src:image_0.jpg" data-bottom="@src:image_10.jpg"/>
```

While *skrollr* is able to interpolate the image filename, it will eventually produce floating point numbers — which doesn't make sense here of course. I needed the interpolated values to stay integers, so they need to be rounded. I introduced `°` as another modifier (similar to `!` for preventing interpolation altogether) for exactly this purpose:

```html
<img src="image_0.jpg" data-top="@src:°image_0.jpg" data-bottom="@src:°image_10.jpg"/>
```

This will result in `src` values like `image_1.jpg`, `image_2.jpg` and so on. Also, it should be easy to implement additional modifiers in a similar fashion.