Added Service#keep_alive? method

[apainintheneck]

• [ x] Have you followed the guidelines in our Contributing document?
• [x ] Have you checked to ensure there aren't other open Pull Requests for the same change?
• [x ] Have you added an explanation of what your changes do and why you'd like us to include them?
• [ x] Have you written new tests for your changes? Here's an example.
• [ x] Have you successfully run brew style with your changes locally?
• [ x] Have you successfully run brew typecheck with your changes locally?
• [ x] Have you successfully run brew tests with your changes locally?

---

This PR is related to the kill command that I'm currently working on adding to brew services (over here). As a part of that I'd like to be able to see if a service has been marked as keep_alive. Currently, there is only a setter for that in the Service class so I added a getter.

[Bo98]

Thanks!