Add support for `flatten` and `background`


#### flatten()

Flatten transparent images before processing. Defaults background color to black. Use `background` to set the background color.

#### background(color) or background(r, g, b)

Set the background color when flattening transparent images using `flatten`. Pass any valid CSS color as `color` or numbers in the range of `[0, 255]` for `r`, `g`, and `b`.

Great feedback and I was wondering about that too! The current implementation looks like this to follow the existing conventions but I will discuss this with the author.