Support film/movie/video entry type

[dcpurton]

See https://sblhs2.com/2016/10/06/citing-films/

There are a few issues

• short forms or editortype (dir., prod., etc) in citations
• strings in bibliography should be "Directed by", "Produced by", etc.
• the "Feature Commentary" example is basically impossible to produce at present
• support revdeditortype