Streaming reads

Hi! Thanks for writing this lib!

Have you thought about the case of streaming reads? It seems the API is centered around `Mp4Reader::read_sample(track_id, sample_id)` which in turn delegates to `Track::read_sample`. Reading the code it seems the latter assumes a `reader.seek(SeekFrom::Start(sample_offset))?;`

With very large files, or continuous TS-streams, it's not necessarily practical to assume the whole things can be wrapped in a BufReader.

Would it be possible to extend the API somehow to have more of a "read-next-sample-whatever-it-is" approach? Happy to contribute a PR.