Add smell for nil checks

#8 
I hope you like this commit better. Feel free to suggest changes.

Very weird thing about the config/defaults.reek. I don't know why it gets rearranged.
On my system the file looks like the old one but with the added settings for the nil_check at the bottom. But when I upload the file to here, it gets rearranged.. Is it ok? Or should I try to get the bottom of this?