Optional kill signal handler registration

I have a use case where the spock event loop is not running in the main thread.
Spock currently assumes it's in the main thread by always calling signal.signal -- I have introduced a setting called "killsignals" for the Event plugin to avoid that registration in cases where spock is running within a larger application that performs its own thread management.
The change is meant to be non-invasive, so everything should be the same for people who run spock in the main thread.