Hoodie plugins - update package.json

We're now going to use a hoodie.plugins property to determine which plugins to load. When that feature lands, we'll need the command line tool to update package.json when `hoodie install` and `hoodie uninstall` are called. I don't think there will be any adverse effect to existing projects if they have their hoodie.plugins property updated (it just won't be looked at by the current hoodie-server code).