Added superClass option. Fixes #1

[Avasam]

Updated documentation and tests (I noticed checkDestroy was missing from the readme as well). Ran the linter and did my best to follow what seemed to be the code style.

#1 (comment)

Adds the option to specify a class that components extends from which already implements the unsubscribe ngOnDestroy pattern.
Since the super class won't extend itself, this keeps everything safe. I've also considered checking for any super class by default (same logic where if the super class doesn't implement the pattern, linting would fail, just in a different location). But decided against it in the end, since someone could extend a class they don't have control over, or isn't linted. If you think it would be useful, I can add that functionality.

[Samuel-Therrien-Beslogic]

Note this doesn't detect calls to super.ngOnDestroy() inside a sub-component's own ngOnDestroy. And will hence raise the errors unsubscribe.next() not called. and Subject unsubscribe not a class property. for rule rxjs-angular/prefer-takeuntil. It shouldn't be too complicated to add support for it using the exact same strategy.

[Samuel-Therrien-Beslogic]

@cartant Could you take a look at this? I don't think anything's missing actually.

[Samuel-Therrien-Beslogic]

I was given some time at work to finish this PR to resolve a handful of false positives in our projects as per my previous comment.