Look in entire document for Schema.org metadata not just head

[westonruter]

The JSON-LD script elements can appear anywhere in a document, not just the head. This fixes the detection to account for that so that when another plugin outputs Schema.org metadata, the AMP plugin won't output a duplicate.

Fixes #1659

[kienstra]

Approved

Hi @westonruter,
This looks good. I also saw that the plugin doesn't output Schema.org metadata if it's already in the document, like in the footer:

And if the metadata isn't already present, the plugin still outputs it like before.