Rubocop: Add a cop to prevent `-> (` lambda format

[astorije]

As discovered in https://github.com/rmosolgo/graphql-ruby/pull/313:

• Both -> (arg) { ... } and ->(arg) { ... } are supported in Ruby 2+ (notice the space between -> and the parenthesis.
• -> (arg) { ... } is not supported in Ruby 1.9.3 and the interpreter reports:

syntax error, unexpected tLPAREN_ARG, expecting keyword_do_LAMBDA or tLAMBEG (SyntaxError)
      -> (arg) {
          ^

It seems reasonable to me to enforce ->(arg) { ... } in style guides. After all, all examples in Rubocop's doc or your Ruby style guide use this format consistently.

This is very valuable when developing a library that supports both versions 1.9.3 and 2+.

Looking forward to hearing your opinion on that.

[bbatsov]

While I don't care at all about 1.9.3 compatibility, I agree that we should stick to the style without a space for historical reasons. I have some memory that at one points we added some similar check to RuboCop, but I might be mistaken.

[savef]

I like the idea of a cop to keep this consistent, but I would like the style to be configurable as I prefer the style with the space.

[bbatsov]

Fine by me.

[swcraig]

I have a working cop for this, and will commit soon.