we can now specify plugins which need to be imported before others

This was discussed an an earlier issue, though that discussion was before we made Sideboard run out of a single virtualenv.

As a plugin developer I want to have one plugin import from another, which mean we need to be able to configure an import order, I did by adding a config option.
This was an unrelated change which I meant to make when I made Sideboard run out of a single virtualenv.