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.  There are too many problems, starting with the arity.

Although this mimics the language `if-else` semantics, it's still about data-transformation and not side-effects, so users should have to decide what transformation is necessary in the `else` case.  Ramda should not be in the business of guessing that for them.