Allow to set encoding config for compass filter.

This allow to set the default and external encoding for compass filter. That help to fix encoding issues ([like this](https://github.com/chriseppstein/compass/issues/377)).
Even it is solvable by addin `@charset "utf-8"` I don't wan't to modify third party libraries that rely on `encoding = "utf-8"` or `Encoding.default_external = "utf-8"` in the config.rb.