Allow to set custom DocumentResolver

[filip26]

Is your feature request related to a problem? Please describe.
HttpLoader can load only RDF and JSON-LD documents.

Describe the solution you'd like
Allow to set custom DocumentResolver enabling custom document loading.

Describe alternatives you've considered
Default DocumentResolver could be exposed and configurable.

[skodapetr]

DocumentResolver is used in the DefaultHttpLoader, FileLoader, and ZipResourceLoader.
Those are created before the JsonLdOptions object, as a result, we can no directly set resolver using JsonLdOptions.

We could:

• Split DocumentResolver into interface and DefaultDocumentResolver.
• Add constructor to Loaders which would allow to set custom resolver.
  That would allow users to provide custom implementation with the added cost of manual SchemeRouter constrcution.