Implemented argmin & argmax for Series

[itholic]

This PR proposes Series.argmin & Series.argmax.

>>> kser = ks.Series(
...     {
...         "Corn Flakes": 100.0,
...         "Almond Delight": 110.0,
...         "Cinnamon Toast Crunch": 120.0,
...         "Cocoa Puff": 110.0,
...         "Expensive Flakes": 120.0,
...         "Cheap Flakes": 100.0,
...     }
... )
>>> kser.argmin()
0
>>> kser.argmax()
2

[itholic]

Addressed comments.
Thanks, @ueshin !

[ueshin]

LGTM.

[ueshin]

Thanks! merging.