Leverage Rust's Traits to create a more dynamic API

It would be awesome if `stego` could use traits to allow for custom encodable/decodable (source) datatypes along with custom host types.

I'm thinking an API similar to the following:
- `Source` trait is for encodable/decodable (hidden) data types.
- `Host` trait is for host files, like images/video/audio files that seem inconspicuous.

We could change the current `LSBStego` implementation to take in two type definitions. A `Host` and a `Source` then the `LSBStego` is the glue between the `Source` / `Host`.