processor options

Recently, I tried to use [postcss-url](https://github.com/postcss/postcss-url) and found out, that it is necessary to set up the `to` option to the [processor](https://github.com/postcss/postcss/blob/master/docs/api.md#processorprocesscss-opts). Looks like it is required to copy files. Unfortunately, we don't provide any possibility for that.

Haven't you experienced something familiar?
