Add support for multiple sources

Currently, Contentlayer looks for a single exported method that is pulled from one particular source. We've received requests to support more than one source within the same configuration. I'm working to get more information on specific use cases.

## Considerations

What are the challenges with this approach?

## Design

What would the API for this look like? Perhaps you could define sources separately and then reference the source at the document level? And include the option for a default source.

Napkin sketch:

```js
const ctfl = makeSource({ type: 'contentful', accessToken: /* ... */ })
const local = makeSource({ type: 'local', path: /* ... */ })

const Page = defineDocumentType({ source: ctfl })
const Post = defineDocumentType({})

export default {
defaultSource: local,
documentTypes: [Page, Post]
}
```