Add a second argument to #document_partial_name for the current kind of partial we're rendering

This allows downstream implementors to override the calculated
 #document_partial_name on a per-partial basis. E.g. if you had a
heterogeneous index with some records with MODS and MARC data, you could
choose the appropriate rendering format for those records independent of
the regular format.
Merge pull request #831 from projectblacklight/partial-name-with-base-name-arg

Add a second argument to #document_partial_name for the current kind of ...