Support literal reports in other vocabularies

[gimsieke]

Not everything that a reporting language reports will be mappable to XVRL (see #9).

In addition, information extraction from “native” reports might be deferred until later.

There may be non-XML reports in JSON, Turtle, etc. So in addition to the content type of the schema (#4), we also need to be able to convey the content type of the report (application/json, text/turtle, application/vnd.openxmlformats-officedocument.spreadsheetml.sheet, etc.). It should be indicated by another attribute when reports are included with a base64 encoding. For text and JSON formats, the default XML serialization of XVRL can store them as a text node with the usual XML escaping, or in CDATA sections.

It should also be possible to link to external reports (with no guarantee that these links will resolve everywhere, all the time).