Update: Add support for plugin default configuration (fixes #1358)

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.
I like this idea. It would be even simpler to implement