DocumentLoader

import documentLoader from 'fontoxml-remote-documents/src/documentLoader.js'

The DocumentLoader manages and de-duplicates load requests for documents and adds corresponding instances of DocumentController to the DocumentsManager.

Documents can be referred to either by their documentId or by an URL relative to some other document. In the latter case, resolving occurs in the CMS, but we can deduplicate requests by checking the documentId returned from such requests.