Unify <link rel=image> and <link rel=features> into a single value, differentiate on @type

[prushforth]

The rel values image and features are intended to imply that the entire extent of the map is covered by the link template, and will return a response of Content-type: image/* in the former and text/mapml in the latter.

In Testbed 15, we added recognition of the type attribute to the link rel="tile" element (type already exists in the HTML link element), and the polyfill looks for image/* or text/mapml to function appropriately to the media type that is expected to be received for each tile. There are probably scenarios that aren't covered properly by how the polyfill behaves; maybe we can specify that further in the future.

However, it seems appropriate that the media type be factored out of the link relation in that way, and maybe this is a good opportunity to deprecate the image and features relations and unify them into rel="map" or perhaps rel="extent", while simultaneously requiring a <link type="..."> attribute to determine the content processing that ensues.

What do you think?