Wraps a function with a function of a sertain arity.
npm install arity-n
```js function fn(a, b, c, d) { }
var arityN = require('arity-n'); var newFn = arityN(fn, 3);
newFn.length; // => 3
var arity4 = require('arity-n/4'); var newFn = arity4(fn);
newFn.length; // => 4
// Max arity is 5. var newFn = arityN(fn, 7);
newFn.length; // => 4 ```