Iterator-based parser

This idea was mentioned in #9 and will greatly improve the usability of the crate by providing an iterator-based parser that will parse new messages from e.g. streamed/received bytes when `\r\n` ending is present.

- Extend with bytes (similar to `Iterator::chain`)
- Extend with a single byte (optional)
- `try_next()` try to parse the currently collected bytes (by searching for an ending - `\r\n`)

Enhancements:
- Track both the ending `\r\n` but also the start of a message `!` (for AIS messages) or `$` (for all other) in the iterator.