Safer way for PostCSS to consume PostCSS object as plugin

Because project like grunt are doing some shitty cloning for option,
the test ìnstanceof PostCSS` can return false, even if the object has
been cloned from a PostCSS instance.
To avoid this, we need to tolerate object that have array `processors`.

Ref https://github.com/nDmitry/grunt-postcss/issues/8