Implement custom document loader option

[BigBlueHat]

Something similar to https://github.com/digitalbazaar/jsonld.js#custom-document-loader would be great. Inspired by #153

[BigBlueHat]

Heh...it may actually be here already! https://github.com/digitalbazaar/pyld#document-loader

@drbh could something like that be used for your IPFS document loading needs?