Determine how to specify outputting a different public key format

[dlongley]

We have API changes to properly multiple key types (as identified by multibase-multikey headers) in #55. We should figure out how we want to layer using different public key formats on top of it -- or decide that it's not appropriate to do that at all and either:

1. A different driver instance should be created with different fromMultibase plugins, OR
2. Presume that cryptosuites should allow all the different formats they accept and perform the necessary conversions.

[dlongley]

Some approach for this was taken in #47, but that PR built on a faulty API (that couldn't properly support multiple key types) so it also needs changing. It's unclear what we can preserve from #47 since it's doing a lot -- it needs to be split into smaller pieces that we can pull in.

[dlongley]

We should add examples to the README showing how to use ed25519-multikey. (In addition to figuring out the above).