Provide an API for user defined metadata

[scriptjs]

To enable broader uses of files in a hyperdrive, it requires an API to enable user defined metadata to be added and extracted from files.

[ghost]

This feature can be done outside of hyperdrive itself with hyperdrive-index. For example, I'm indexing jpeg markers with hyperdrive-jpeg-index and soon I'm also going to write thumbnails out into a separate hyperdrive archive.

[scriptjs]

Indexes are definitely a good thing. I don't think the addition of this API is that expensive but its really a decision in the end whether or not to implement it.