[manifest] Support for deep integrations, e.g. OpenSearch (#421)

Listing deep integration endpoints in the manifest could allow user-agents to offer customized UI for specific actions. E.g. OpenSearch defines a [URL syntax for search results](http://www.opensearch.org/Specifications/OpenSearch/1.1#OpenSearch_URL_template_syntax) based on query terms provided by the user.

Instead of requiring individual `<meta>` tags for all these, a single manifest could house all such endpoints. 

Strawman format:

    "endpoints": [
      {
       type: "search",
       label: "Search the Web",
       url: "http://example.com/search?q={query}"
      },
      {
       type: "search",
       label: "Search Videos",
        url: "http://example.com/search?corpus=videos&url={query}"
      }
    ]


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Reply to this email directly or view it on GitHub:
https://github.com/w3c/manifest/issues/421