v1.18.0

Enhancements:

• #961: Add zapcore.BufferedWriteSyncer, a new WriteSyncer that buffers
  messages in-memory and flushes them periodically.
• #971: Add zapio.Writer to use a Zap logger as an io.Writer.
• #897: Add zap.WithClock option to control the source of time via the
  new zapcore.Clock interface.
• #949: Avoid panicking in zap.SugaredLogger when arguments of *w
  methods don't match expectations.
• #943: Add support for filtering by level or arbitrary matcher function to
  zaptest/observer.
• #691: Comply with io.StringWriter and io.ByteWriter in Zap's
  buffer.Buffer.

Thanks to @atrn0, @ernado, @heyanfu, @hnlq715, @zchee
for their contributions to this release.