Structured attribution

Nobody has claimed this yet.

As explained in https://github.com/mapbox/mapbox-gl-native/issues/2723#issuecomment-239383427, native map controls need attribution information in a format other than HTML source code. We should allow the `attribution` property to be set to something more structured that can be converted to the appropriate data structures on the client side. For example, it could be an array of objects, each with a required `title` property (interpreted as literal text) and an optional `url` property (for linking):

``` json
{
"attribution": [
{
"title": "© OpenStreetMap contributors",
"url": "http://www.openstreetmap.org/about/"
},
{
"title": "Creative Commons Attribution-ShareAlike 2.0",
"url": "http://www.openstreetmap.org/copyright/"
}
]
}
```

Other considerations:
- Should the `title` property be localizable? If so, perhaps it should be set to an object mapping ISO 639 language codes to literal strings.
- Mobile clients are often space-constrained. In the example above, it may be advantageous to abbreviate the two items as “© OSM” and “CC BY-SA”. These abbreviations could go in a `short_title` property beside the `title` property.
- Should other formatting be allowed? The current specification doesn’t disallow any HTML tags, so it’s theoretically possible to boldface text or embed images. While such content may be feasible in Web clients, native clients may have difficulty displaying it.
- Perhaps there should be a separate `feedback` property beside the `attribution` property. Many TileJSON documents hosted by Mapbox include an “Improve This Map” link alongside copyright statements. Native clients may want to open an alternative map feedback tool (mapbox/mapbox-gl-native#6001); as things stand, they’d have to look for and omit a specific URL. In space-constrained environments, the client may choose to omit feedback items from the attribution UI to ensure that all the legally required copyright statements are visible.

/cc @kkaefer @jfirebaugh

Start by reading the TileJSON specification and the linked discussion about native map controls. Resolve the structured attribution schema, including localization, short titles, formatting, and feedback handling, then document the agreed format and its compatibility expectations.

Written by the indexing model from the issue text.