ENH: Add Styler.where
- [x] closes #16255
- [x] tests added / passed
Added `test_where_subset` in `pandas/tests/io/formats/test_style.py`.
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry
It makes the redundant style condition simple.
```python
df.style.applymap(lambda val: 'color: %s' % 'red' if val < 0 else 'black')
df.style.where(lambda val: val < 0, 'color: red', 'color: black')
```