VIS: plot method cannot handle new 'C0'-like colors (matplotlib 2.0)
matplotlib 2.0 introduced a few 2-character color abbreviations: C0, C1, C2, ... to refer to the colors of the current color cycle 
(very handy! this way you can refer to a color of the color cycle without hardcoding it, and use it as a shortcut for `color='tab:blue'` if you want the new blue instead of the ugly `color='b'`)

But, it seems pandas cannot handle this at the moment, as it tries to interpret it as multiple colors:

```
df = pd.DataFrame(np.random.randn(10,3), columns=['a', 'b', 'c'])
df.plot(color='C0')
```

gives `ValueError: 'C0' can be parsed as both single color and color cycle. Specify each color using a list like ['C0'] or ['C', '0']`

I know the error message already gives the solution (using `color=['C0']`), but I think it would be nice to special case those to have to working by default.