AVRO-1379. C: avro_file_writer_append_encoded() function

Allows you to append to a file an Avro value that you already have the
binary encoding for.  Contributed by Mark Teodoro.


git-svn-id: https://svn.apache.org/repos/asf/avro/trunk@1551926 13f79535-47bb-0310-9956-ffa450edef68