This adds magic number support, and tries to address real-world
complications such as:
- The same extension might mean two or more different formats
- The extension might lie about the format
- Some files don't have magic numbers
- Some magic numbers are found in weird places in files

- register packages that should be loaded before attempting to work
with a particular file format.
- add utilities to retrieve or skip over the magic bytes
- write a lot of help!