support json formatted metadata (eg: schema.org) in `runCheck` function

[jeanetteclark]

To support schema.org, it would be nice if this R package supported both XML metadata and json metadata formats. This will involve refactoring runCheck so that it can accept a json document in addition to an XML doc, and be able to run the jq to select the correct metadata field just as the xpaths work for the xml documents.

the jqr package will be key here.