Support for arrow function when refactoring setInterval

[Siorki]

ECMAScript 2015 / ES6 introduces arrow functions. They can shorten (as in less bytes, think js1k) the definition of a main loop :
setInterval ( function() { /* contents */ }, 20)
becomes
setInterval ( () => {/* contents */ }, 20)

The option to refactor the call to setInterval() should support that new syntax.

[Siorki]

Not an emergency : it is still possible to write code using the old syntax, and the result after using the option will be the same. This is more convenient than required.

[Siorki]

Done. See also #56 regarding ES6 support for that module.