improves usability of style calls on axis=1 by automatically creating…
- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `git diff upstream/master | flake8 --diff`
- [ ] whatsnew entry

… the IndexSlice same as it does for axis=1

```
df = pd.DataFrame(index = ['x', 'y', 'z'])
df['a'] = [1,2,3]
df['b'] = [4,5,6]
df['c'] = [7,8,9]
df

df.style.background_gradient(subset='x', axis=1)
```
# otherwise this throws some vague exception. seems like if you pass in

axis this should just work

`df.style.background_gradient(subset=pd.IndexSlice['x',], axis=1)` #same
as this
