Add a feature which requires hooks handler to react to SIGTERM

[honzajavorek]

Taking this out of https://github.com/apiaryio/dredd-hooks-template/pull/65/files#r290878471 where @kylef proposes:

Maybe in this case if a process doesn't gracefully exit from SIGTERM that should be a test failure too (instead of having to SIGKILL it).

This is a proposal to add a feature, which ensures the hooks handlers can react to SIGTERM correctly (SIGINT could be tested too).

Downsides of introducing this is that it would be slightly harder to write a hooks handler (the author already needs to know TCP, now they'd need to know what signals are and how they work) and that SIGTERM (or any alternative to it) doesn't exist on Windows, which would make it complicated for those either aiming for Windows support (#21) or developing the hooks handler on Windows (no way to test locally that it works).