Support layouts

[tkawa]

This PR fixes #7. it probably keeps the fast performance.

[tkawa]

This is useful for putting on a common envelope. in case of Collection+JSON:

{ "collection" : 
  {
    "version" : "1.0",
    ...
  } 
}

in case of JSON-LD with schema.org:

{
  "@context": "http://schema.org/",
  ...
}

etc.