add --kill timeout option

[wclr]

I sometimes found a situation when child process failed to react on SIGTERM and kill itself in a timely manner (even if it handled SIGTERM), so it is useful to have ability to force child kill after a timeout.

If you ok with this feature, then some questions to answer:

1. how to name the parameter --kill or more explicit --kill-timeout?
2. need to add docs.
3. not sure if tests are needed for this.