Add loader option for custom context loader logic

[bergos]

jsonld.js and jsonld-streaming-parser.js offer an option to use a custom loader code to fetch a context given as URL. This option should be forwarded.

A very common use case could be caching on the file system or a context bundled with a web application. An additional option could be added to define a URL + file path map or URL + object map. If that option is given, a loader for the map will be used.

jsonld.js and jsonld-streaming-parser.js call the option documentLoader, but actually it's only used for the context. I would propose to call the option contextLoader.

The map option would be accordingly contextMap.