Util.operators: allow variadic methods to work with arrays

Variadic util operators, such as _.add(), can now work with argument arrays.

e.g. `_.add([1,2,3]) === 6`
The `_.flatten(arguments, true)` would only be a shallow flatten, but I can change it to `var numbers = _.isArray(arguments[0]) ? arguments[0] : arguments` if you like.