Add `including()` methods to `Document`

Document gains including() method that allows you to easily add includes to the document (or more accurately create a new Document with the given includes added to it). There are actually two variants of this method:

1. Take a Document with IncludeType of NoIncludes and map it to a Document with the given includes in whatever new type you would like.
2. Take a Document with IncludeType other than NoIncludes and add more of the same type of includes to it.