(Optionally) allow "self" links to add to cache

[geraintluff]

Setting in Jsonary.config to turn off.

Should be linked somehow - i.e. update of one updates the other. Could we simply hard-wire data object together? That seems like it might disrupt things (e.g. child data can have different document).

Alternatively, could just maintain list of claimed URLs (we already do this for fragments), so once you've fetched the composite data first, then a call to "fetchData(...)" would return part of that document.

Only trust parent URLs, so could we search through the cache to find possible candidates, avoiding a central registry?