Add a Stop command to generic module

[sthibaul]

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