Extract examples Improve example checking and generate example files

[gkellogg]

This update makes use of improved features in extract-examples.rb to process all files, rather than simply doing syntactic validation, and to compare with expected results. Expected results have @data-result-for referencing the title of the source example. Expected results can also be in script elements, which don't show up in the rendered documentation.

The extract-examples script was also updated to extract JSON, as well as generate Turtle/TriG and YAML for most examples; these are updated using rake examples.

Eventually, multiple formats will be associated with each example, so the identification mechanism may change.

[iherman]

@gkellogg I do not really have the background for a formal review: I have never looked into the detail of this before. At this stage, from the WG's point of view, this should just be an input to the WG that we will have to formally look into, without any reference to the previous stage (which the WG has never looked into yet anyway). Ie, I would say, just merge it, and then, at some point, the WG will have to look it it overall.

This is also valid for the two other PR-s of a similar vein: w3c/json-ld-syntax#49 and w3c/json-ld-api#23