Load (and render) child plugins correctly for inherited plugins.
Before the changes in this commit, child_plugin_instances of inherited plugins would be None; loading all descendant plugins and building the tree resolves this issue.
Not entirely sure if this might break things or cause insane slowdowns, (I couldn't detect any errors myself) but it "works for me".
Implementation may not be the best either.
Merge pull request #4 from jorisvddonk/master

Fixed inherited child plugins not being loaded and rendered correctly