add support for preexec_fn

[dz0ny]

This is used to change user and group of running process

[amoffat]

seems simple. if someone gets to it first, it would probably go here

https://github.com/amoffat/sh/blob/master/sh.py#L1287

and do something like

preexec_fn = self.call_args["preexec_fn"]
if callable(preexec_fn):
    preexec_fn()

and add the name "preexec_fn" here https://github.com/amoffat/sh/blob/master/sh.py#L669 with a default of None

then documentation and tests