Fix the way spinsv kills the process

With this patch spinsv sends SIGTERM followed by a configurable grace period before a SIGKILL. User defined kill command precedes above if given.

Previous behaviour would run a custom command given by the user without any guarantee that the process actually gets killed.

The grace period, defaulted to 7 seconds, can be configures via --kill.grace-period-seconds optional argument.

Patch by @omidaladini @seeemilyplay‎!