API: Add Styler.where

#### Code Sample, a copy-pastable example if possible

```python
# Instead of 
df.style.applymap(lambda val: 'color: %s' % 'red' if val < 0 else 'black')
```

It would be

```python
df.style.where(lambda val: val < 0, 'color: red', 'color: black')
```