Implement a check for stabby lambda params parentheses

[bbatsov]

We have to implement something similar to https://github.com/bbatsov/rubocop/blob/master/lib/rubocop/cop/style/method_def_parentheses.rb

Probably it should be configurable, but as long as it flags code like this, that we'll be good enough for me:

l = ->a,b,c { ... }