Parse unknown metadata and write metadata

[jessa0]

This is a duplicate of alfg#91 for internal review.

The meta box is very open-ended, leaving itself open to extension by future specs. This PR:

1. parses the Hdlrbox as required by the spec for the meta box
2. only parses iTunes metadata when the handler type is mdir (as seen in `big_buck_bunny_metadata.m4v)
3. makes MetaBox into an enum, to allow for other handlers in the future (like requested in question: MOV Quicktime and MP4 metadata alfg/mp4-rust#66)
4. allows parsing MetaBox in more positions as per the spec
5. writes metadata out if present