API: .argmax should be positional, not an alias for idxmax
In https://github.com/pandas-dev/pandas/issues/6214 it was reported that `argmax` changed (I think since Series didn't subclass `ndarray` anymore). It's sometimes useful to get the index position of the max, so it'd be nice if `.argmax` did that, leaving `idxmax` to always be label-based.

```python

In [1]: import pandas as pd
pd.Ser
In [2]: s = pd.Series([1, 2, 3, 0], index=['a', 'b', 'c', 'd'])

In [3]: s.idxmax()
Out[3]: 'c'

In [4]: s.argmax()
Out[4]: 'c'
```

I would expect `Out[4]` to be `2`. The current workaround is `np.argmax(x)`, which is OK, but doesn't generalize to DataFrames well.

We could deprecate `.argmax` in 0.21, pointing users to `.idxmax`.  Then later we can change `.argmax` to be always be positional.

---

Update: We've deprecated `.argmax/min`. Need to change the behavior next.