ensure opts from writer is used to generate hypercore

[m4gpi]

Enables per-hypercore opts to be used over default set by multifeed instance. We need this in kappa-drive for an update to hyperdrive, see here:

https://gitlab.com/coboxcoop/kappa-drive/-/merge_requests/4/diffs#a77b15c0d64cf6111e025645e562faf8ccabb9d7_427_427

We're using multifeed's writer function to generate feeds, then creating a trie and hyperdrive out of those returned feeds. This custom opts allows us to create a trie with { valueEncoding: 'binary' }, which is different to the other's feeds encodings.

[hackergrrl]

Thanks for the patch! Would you be open to using opts.hypercoreOpts or opts.hypercore for the opts to be passed down to the hypercore? Since writing this module I've become a bigger fan of namespacing pass-down opts with a key, to avoid passing down things by mistake.

Could you also add a note in the README about this new opts?