Support JSON-LD in snippet extraction

[blakeembrey]

Currently it's being scrapped when extracting a snippet, but it usually provides the richest metadata and types.

• Product
• Event
• LocalBusiness
• Organization
• SoftwareSourceCode
• Article
• Movie
• BlogPosting
• MusicRecording
• VideoObject
• ImageObject

[blakeembrey]

Also microdata.

[blakeembrey]

This should be a lot easier now that RDFa, microdata and JSON-LD are all being normalised to the same shape.