Add support for compacted form of json-ld when using RDFaExtractor.

[olegbonar]

RDFaExtractor already supports expanded mode via expanded parameter but it seems that currently there's no way to pass it when using "All-in-one" extraction via extruct.extract
It'd be nice to add that support either by introducing a constructor argument to RDFaExtractor or by adding keyword arguments (see LxmlMicrodataExtractor for reference)

[lopuhin]

Let's keep this open, I think the feature makes a lot of sense.