feat(if): add conditional "if" function

Adds a conditional function that can be used for the some of the following situations:

```
R.each(R.if(Array.isArray, function() {
    // Perform operation only on arrays in our list
}));

// Flatten all arrays in the list while keeping all other value types
var flattenArrays = R.map(R.if(Array.isArray, R.flatten, R.identity));
```
Not too sure. It's optional in `Racket`s [`cond`](http://docs.racket-lang.org/reference/if.html#%28form._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._cond%29%29)