toRdf added

[Gnomus042]

toRdf function proposal, which should resolve the issue #22

[Janpot]

I'm hesitant releasing a feature without tests. If you're up for it you can add test logic after this:

microdata-node/test/suite.spec.js

Line 36 in a0473b6

await assertEqualRdf(jsonldExpected, jsonldGot, { base: base });

where you run toRDF on the input, convert it to jsonld with jsonld.fromRDF and then use assertEqualRdf on the result, just like the jsonld test.