io: parse metadata through otherData field

[sluongng]

fixes: #8

[omaskery]

This looks good to me. As per your question on the linked issue: I think that complying with the spec and naming it OtherData would be a good idea, so I'll not mark this as approved yet.

I'd recommend leaving the existing public accessor on TefData as Metadata() though to preserve API compatibility, add a Deprecated: ... comment (as described in here), and just add a new accessor to the same data called OtherData(), then if this ever gets a major version bump we can just delete the Metadata() one.

Thank you!