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));
```
Agreed.  Other possible names: `onTrue`/`onFalse`, `truthy`/`falsy`, lots of possibilities.  Even `validator` doesn't really sound right to me.  `test` or `condition` sound better.