Inconsistent kwarg naming between step() and fill_between()

Calling step style is using a different kwarg name in `step()` and `fill_between() i.e.
```
ax.step(x, y, where='post')
```
and
```
ax.fill_between(x, y, step='post')
```

Maybe use `step` in both or pick a better kwarg name usable in both and not conflicting with `where` in fill_between which turns bin filling on/off