Include source and source_name vector_layers keys

Nobody has claimed this yet.

When more than one tileset is composited, vector_layers get merged into a single list. `source` and `source_name` fields are useful for identifying which vector_layers belong to which member of a composite source.

```js
// An example vector layer entry with `source` and `source_name`:
{
description: string,
fields: { [string]: string },
id: string,
maxzoom?: number,
minzoom?: number,
source: string, // Matches id of parent tileset
source_name?: string // Matches name of parent tileset
}
```

This does bring up a bigger question for me. I find the tileJSON spec's ability to successfully describe composited tileJSON to be lacking. I want to know more details about each tileset in my composite. A nested data structure for composited tileJSON that includes the full original tileJSON for each composited tileset would be a much more convenient document to work with, for example.

Start by reviewing the tileJSON specification and the composite-source behavior described in issue #54. Determine the scope for adding source and source_name to vector_layers, and whether nested metadata for each composited tileset is included. Done means the specification clearly defines the supported fields and composite representation.

Written by the indexing model from the issue text.