Remap properties based on context

[warriordog]

The JSON-LD context is permitted to remap / rename properties, and we need to handle this case. This will require the full content of external contexts, so we need to either load it on-demand or require the implementing application to provide it in advance. The latter option is much easier but not technically accurate, as a remote context could still remap properties. Still, this should cover 99% of real-world cases so the proper implementation can wait until #TBD.

Depends on #98, #97, and #95.