Add an option to enable inconclusive cppcheck checks.

This allows Cppcheck to report warnings it's not certain of, but it may result in false positives.
@Fanael Oh, indeed, I had completely misinterpreted this test. I'm terribly sorry, please excuse my thoughtlessness.  

I'll merge the PR then.  Did you decide [how and whether you'd like to appear in CREDITS.rst](https://github.com/flycheck/flycheck/pull/407#issuecomment-45797282)?