Add support for keepClasses option

Hello, thanks for building this useful program.

The readability library supports keepClasses option. It would be nice if readability-cli allowed specifying this option from command line, because source blocks in web pages usually have class attributes which indicates their language, and the classes are useful for adding syntax highlighting on the client side.

Have you thought of adding this option? If not, I may be able to contribute to this project on the feature.