Support for different data sources #2

Currently ptero can only hide data in text. Goal is to refactor the code to be able to easily add new data sources (like images, video, executables etc.) and architect it in a way that it will be easy to add new encoders/decoders for such sources.

Architecture is not yet defined, so this is definitely in scope of this task to provide one. Ptero could be used as a library, so goal is to provide arch for CLI and lib seperately as currently API isn't very good and friendly.