OS X support for automatic mode

Automatic mode currently only works on Linux. An OS X version would also be great.

An OS X-compatible version of [lyner.linux.LiveSourceTarget](https://github.com/rgson/Lyner/blob/master/lyner/linux.py) would be required. The appropriate version could then be chosen using [sys.platform](https://docs.python.org/3.6/library/sys.html#sys.platform).