Implement generic linked data beacon

[cmungall]

Following on from discussion with @micheldumontier @newgene @jmcmurry @balhoff this a.m

We can implement a beacon directly over linked data. If a URL resolves to triples, we can just follow these outwards, gradually building out the graph.

Not clear what strategy should be for CURIEs, just use URLs directly?

[micheldumontier]

@amalic