Allow unused loop variables

[za-creature]

Split from #6

for key, val of someObject
  doSomethingWith(val)

gives me:

✗ #1: Variable "key" is never read ( first defined on line 1).

Is there an option to ignore this? I know I could write this in another way but it's nice to use the for ... of syntax here.

[zmillman]

I think this was also resolved by #39 (use _key as your variable name to match the ignore pattern)