Implement support for 3rd party URI schemes

[njgheorghita]

What was wrong?

from #152

Allow 3rd party URIs to be registered and accepted. Bitbucket, gitlab, gists, .... ipfs-gateway uris (there can be more than one gateway, cloudflair has one). The EthPM spec is intentionally open about the definition of a content addressable URI and thus, there are tons of options out there for people to include the content hash in different URI schemes.

How can it be fixed?

Probably a similar mechanism to the eth-abi library with some kind of Registry implementation that allows new ones to be added with the library supporting the common use case with a default registry with the built-in URIs pre-registered.