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));
```
it is `identity`, but defining it inside the test means that we aren't depending on `R.identity`. So i am fine with this.