WIP: Add support for plugin default configuration

As discussed in #1358 
Everything seems to be working currently. I do not like that we are loading plugins twice every time, once in CLIEngine and once in Config, but couldn't come up with a better way to do this. Also, since plugin return object already uses `rules` property for a different purpose then .eslintrc, I assumed another property on the returned object called `rulesConfig`. Maybe that's not the right way to do this, not sure.
If the plugin exports properties like `env` or `plugins` it also gets merged into the config.
Maybe you should return a new object which only has the `rules` property.