v0.17.0

v0.17.0

• upgraded HarfBuzz to 2.8.2 (#91)
• Buffer.glyph_positions() can now return None during tracing
• enabled DirectWrite shaper on Windows (#77)
• fixed crash when all shapers fail (#83), a RuntimeError exception will
  be raised in this case
• respect return value from message callback function (#51)
• fix crash when accessing Buffer.language before setting it (#89), None
  will be returned for unset language and script