`blend()`

Add a `blend()` function to mix colors. It must implement *at least* all blending modes defined in W3C's [Compositing and Blending Level 1 spec](http://www.w3.org/TR/compositing).

```
blend(source, backgrop = #fff, mode = normal)
```
Reference implementations:
- https://github.com/pdaoust/stylus-helpers/
- https://github.com/heygrady/scss-blend-modes
- https://github.com/less/less.js/
- https://github.com/kevinports/blending-modes
- https://github.com/koteus/hipsterize
- https://github.com/processing-js/processing-js