Option to rename blacklisted method names

Hey - I've added an optional options object that can contain an `autoRename` property.

When supplied this will prepend the [blacklisted](https://github.com/Raynos/observ-struct/blob/master/index.js) properties as per #2 

I've added tests and a note on the readme - let me know if this is alright or if you'd like it in another form (like the option name different or something).

Thanks!