Feature request: camera elevation

Nobody has claimed this yet.

## Motivation

With extruded geometries, there are use cases where the focus should be on a specific elevation.

The most obvious is to focus on a building rooftop with a call to `Map#flyTo`. With more 3D data, users might want to have a control to travel "up" and "down" besides moving N-S-W-E.

## Design

I suggest a new property in [`CameraOptions`](https://www.mapbox.com/mapbox-gl-js/api/#CameraOptions), named `height` or `elevation`, e.g.:

```
map.flyTo({
center: [ 63.5, 10.4 ],
zoom: 17,
bearing: 15,
pitch: 45,
elevation: 100 // assumed meters, or CRS units, as in the extrude height
})
```

### Implementation

I guess the internal camera state shall store the camera elevation somewhere, and modify the 3D transform matrix as appropriate.

This might change the viewport bounds, so the calculations for the tile ranges might change as well (my knowledge of the internals of this project is low, so I don't know if the 3D transform matrix is reused for this purpose).

Any easing function for the camera state (`flyTo` animations and the like) shall also ease and interpolate the elevation.

Start with CameraOptions and Map#flyTo, then trace how camera state feeds the 3D transform matrix and viewport or tile-range calculations. Check how camera easing handles other properties. Done means elevation is accepted, affects the camera and relevant bounds, and is interpolated during flyTo animations.

Written by the indexing model from the issue text.