Fix Safe Mode on unclosed blocks
Release 3.0.6 version
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
Merge pull request #150 from postcss/fix/safer-postcssception

Safer way for PostCSS to consume PostCSS object as plugin
Clean up code
Rename childs to nodes
Remove deprecated APIs