Add an index parameter (in addition to position) for Map#addControl

Nobody has claimed this yet.

## Motivation

Currently, `Map#addControl` enables developers to specify a `position` from the list of `'top-left'` , `'top-right'` , `'bottom-left'` , and `'bottom-right'`, but the chosen `position` determines whether further controls added to said `position` are appended or prepended to controls already added to the `position`. Enabling developers to specify an `index` within the `position` to add the new control would support a wider range of use cases, such as appending rather than prepending (as is currently the default and only behavior) when adding to a `position` of `'bottom-left'` or `'bottom-right'`.

## Design Alternatives

There is currently no way to "correctly" accommodate this -- developers can simply access the `Map#_controlPositions` private class member to re-order the controls, but use of this method likely shouldn't be encouraged.

An alternative suggestion which would require no changes to the GL JS API would be for developers to change the order in which particular calls to `Map#addControl` are made. However, this could be a considerable overhead if developers are porting functionality from other mapping providers which use different default ordering.

## Design

Adding an optional `index` parameter to `Map#addControl` which allows the developer to specify the index at which to add the new control.

### Mock-Up

```
// Add a navigation control to the bottom right.
map.addControl(new mapboxgl.NavigationControl(), 'bottom-right');

// Add a GeolocateControl below the existing NavigationControl, rather than on top.
map.addControl(new mapboxgl.GeolocateControl(), 'bottom-right', 1);
```

### Concepts

There aren't many places in the Mapbox GL ecosystem where an "index" would be of particular important to a developer (beyond the "ordering" of layers, which is more Style Specification-specific). That being said, I believe `index` is a relatively intuitive term for most developers. Perhaps `order` would be a more accessible choice. I'm open to considering other terminology to make the parameter's intention as clear as possible.

### Implementation

In the implementation of `map#addControl`:
```
const positionContainer = this._controlPositions[position];
const childNodes = positionContainer.childNodes;
if (index && index <= childNodes.length) {
const refNode = index < childNodes.length ? childNodes[index] : null;
positionContainer.insertBefore(control, refNode);
else {
// Current logic for adding new control's DOM node
if (position.indexOf('bottom') !== -1) {
positionContainer.insertBefore(controlElement, positionContainer.firstChild);
} else {
positionContainer.appendChild(controlElement);
}
}
```

Start at the implementation of map#addControl and inspect _controlPositions and its position containers. Confirm how the optional index should interact with existing top and bottom insertion behavior; done means indexed controls are inserted at the requested position while calls without an index retain current behavior.

Written by the indexing model from the issue text.