v0.0.19

Notable changes:

• C++ extractor: KYTHE_ROOT_DIRECTORY no longer changes the working
  directory during extraction, but does still change the root for path
  normalization.
• http_server: ensure the given --serving_table exists (do not create, if missing)
• Java indexer: fixes/tests for interfaces, which now have extends edges
• kythe tool: display subkinds for related nodes in xrefs subcommand

Notable additions:

• entrystream: add --unique flag
• write_tables: add --entries flag