Add timeout option to JODConverter.

Original work by https://github.com/documentcloud/docsplit/pull/34 with
modification to not use a default timeout (causing no change from
existing functionality).
Add timeout option for LibreOffice and GM.
Don't freeze reading after timeout and prevent zombies.
Make process output read subject to timeout.

The readlines itself should be very fast. Moving it inside the timeout
ensures that a zombie process, etc, doesn't cause it to hang
indefinitely.