Add watchdog_interval method

[Mr0grog]

This adds a new watchdog_interval method that returns the current watchdog interval in seconds (or 0 if no interval is set). Fixes #6.

A few notes here:

• I tried to keep this simple by just adding a watchdog_interval method and not changing any other code. That said, it might be convenient if watchdog? returned false if disabled and the interval (instead of true) if enabled. That would work fine for most people (since floats are truthy), but might be subtly breaking if someone is literally comparing the return value to true.

• Revised the docs for watchdog? a bit to hopefully make them clearer for users who aren’t already deeply familiar with sd_notify, instead of focusing on under-the-hood bits like environment variables. Happy to change this back if you don’t like that.

• Added tests for watchdog? since there weren’t any.

• Added an example of using watchdog to the README. It’s a little complex! Let me know if you have ideas for making it simpler or clearer, or if I should remove it.

• When I ran Rubocop, I got a huge number of errors, mostly from cops that are probably new since this gem last received a major update. I went ahead and turned most of them off in the config, although I did add required_ruby_version to the gemspec file since that seemed like a good idea. I set the value (>= 2.3.0 >= 2.2.0) based on the GitHub Actions CI job. Let me know if I should change or remove it.

[Mr0grog]

Given how many cops I had to turn off in this PR, it might be better to disable them all here and explicitly turn on the ones you want. I’m not sure which you want, though, so I just turned off all the new and/or error-triggering ones.