Add raw json output to JsonLdExtractor

[Granitosaurus]

Sometimes jsonld schema is prefered in raw json format rather than python dict - this PR implementes as_json kwarg bool to determine whether to return python dictionary or json string.

Some use cases:

1. Use alternative json library
2. Use json.loads kwargs
3. Use object hook functions

My personal use case is to strip away @ keys with object hook:

def strip_jsonld(data):
    return {k: v for k, v in data.items() if not k.startswith('@')}

data = json.loads(script, object_hook=strip_jsonld)

[codecov[bot]]

Codecov Report

Merging #103 into master will decrease coverage by 0.32%. The diff coverage is 83.33%.

@@            Coverage Diff             @@
##           master     #103      +/-   ##
==========================================
- Coverage    87.3%   86.98%   -0.33%     
==========================================
  Files          11       11              
  Lines         457      461       +4     
  Branches       97       98       +1     
==========================================
+ Hits          399      401       +2     
- Misses         52       53       +1     
- Partials        6        7       +1

Impacted Files	Coverage Δ
extruct/jsonld.py	100% <100%> (ø)	:arrow_up:
extruct/rdfa.py	91.3% <60%> (-8.7%)	:arrow_down:

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[Granitosaurus]

Also added this feature to RDFA extractor and updated readme with short examples.

[Gallaecio]

This looks mostly ready. What about adding a test covering the usage of parse_json=False?

[Granitosaurus]

Sorry that it took me so long to attend to this but the tests gave me a bit of an headache :D

Should be good to go now!

[Gallaecio]

TestRDFa.test_wikipedia_xhtml_rdfa_no_prefix seems to be failing after your changes.

[Granitosaurus]

TestRDFa.test_wikipedia_xhtml_rdfa_no_prefix seems to be failing after your changes.

It has been failing master for me too; I actually update the fixtures in this PR to prevent it failing but the test just seems to be flawed in some sense - it keeps either breakin on travis or on my machine locally. :man_shrugging: