Add terminateTimeout option

[Jonham]

Forked from fluent-ffmpeg#1292 by @ricfio

Add terminateTimeout option (defaults to 20 ms) to tuneup the workaround in output stream 'close' event:

          // Don't kill process yet, to give a chance to ffmpeg to
          // terminate successfully first. This is necessary because
          // under load, the process 'exit' event sometimes happens
          // after the output stream 'close' event.