Add a Stop command to generic module

**Is your feature request related to a problem? Please describe.**
Currently the generic module just kills the child on stop. But if the synth is actually a web service it'll continue speaking

**Describe the solution you'd like**
We could add a GenericStopSynth parameter to tell how to stop the synth

**Describe alternatives you've considered**
The generic module could send SIGTERM instead of SIGKILL and the child catch it, but that'd not work with mere `curl` calls and such