Make `documentLoader` optional

[BigBlueHat]

Related to #41

I'm working inside Apache CouchDB's JS environment and it (on purpose) has no outbound HTTP request capability. As such, I'm having to write a stub documentLoader to fake that process.

It might be helpful to be able to set documentLoader to false or otherwise disable that functionality or ship with a simple stub documentLoader that references a .contexts = {} list like the one described in the README.

This may also be helpful when the developer doesn't want to do any additional requests to build/parse/change the JSON-LD.

Thanks!

[dlongley]

+1 for providing an optional "local only" document loader like the one in the README.

[dlongley]

Instead of providing a local context loader I think the README should be updated to reference another module that provides a document loader with the features mentioned. I'll leave this issue open until such a thing exists and we reference it in the README (or until we decide this is a bad idea instead).