Allow to retrieve documents as raw unformatted text

By setting the URL parameter 'raw' a document for which a formatter is
registered can be retrieved as raw unformatted text, e.g.:

  /xdocs/project/external%2Fopenssl/rev/stable-1.3/docs%2Ffaq.md?raw

The 'raw' parameter cannot be used for binary files.

Change-Id: I9579b4302b1fd5a8efdb1661bd03c206d01f96f3
Signed-off-by: Edwin Kempin <edwin.kempin@sap.com>