Added a modifier mechanism for rounding interpolated values

[jkphl]

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:

<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:

<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.

[sergeylukin]

Thank you @jkphl, it works! I have similar requirement when dealing with element's height. Tiny whitespace may appear below the element if floating point number is used in height value so rounding does the trick. This PR works great for me.
@Prinzhorn Is there a reason this PR is not merged? Are you planning on merging it / implementing similar functionality?
Thanks