Explore ways to support site-specific plugin dependencies

[jbmorley]

It would be useful to be able to add site-specific plugin dependencies to allow site plugins to do more complex and interesting things. I recently wanted to use the pick module and sadly this isn't currently built into InContext, meaning it's not possible to use. Ideally doing this would not 'taint' the Docker image or cause it to be rebuilt (though I question whether that's possible).