registerExtension support

[RangerMauve]

It'd be nice to have a way to manage extensions for connections in a corestore in the same way we can manage extensions for connections related to a specific hypercore. At the moment, it's a bit of a pain to use protocol-level extensions with corestore since you need to manually manage connections in addition to corestore managing them.

Would a PR that adds the same registerExtension API as hypercore be welcome? 😁

cc @Frando Does this seem like something that'd make corestore more appealing for kappa?