Support discovery

From http://www.oembed.com/

oEmbed providers can choose to make their oEmbed support discoverable by adding elements to the head of their existing (X)HTML documents.

For example:

<link rel="alternate" type="application/json+oembed"
	href="http://flickr.com/services/oembed?url=http%3A//flickr.com/photos/bees/2362225867/&format=json"
	title="Bacon Lollys oEmbed Profile" />
<link rel="alternate" type="text/xml+oembed"
	href="http://flickr.com/services/oembed?url=http%3A//flickr.com/photos/bees/2362225867/&format=xml"
	title="Bacon Lollys oEmbed Profile" />

The URLs contained within the href attribute should be the full oEmebed endpoint plus URL and any needed format parameter. No other request parameters should be included in this URL.

The type attribute must contain either application/json+oembed for JSON responses, or text/xml+oembed for XML.

I can see how this is going to be tricky because oembedprovider.module passes around an absolute URL and a PHP function callback. If we could pass around a menu path instead or in addition, hook_init() could check the list of providers for local paths and if any matched the current one, add a link element to the HTML head.