Added p:cast-media-type

Per Vojtech's comment on the pull request:

  https://github.com/xproc/specification/pull/98

I have added a p:cast-media-type step:

https://ndw.github.io/specification/langspec/general-docs/head/#c.cast-media-type

                                        Be seeing you,
                                          norm

-- 
Norman Walsh
Lead Engineer
MarkLogic Corporation
Phone: +1 512 761 6676
www.marklogic.com