ENH/BUG: support TimedeltaIndex plotting
This raises

```
s = Series(range(5),pd.timedelta_range('1day',periods=5))
s.plot()
```

This will show the timedeltas with a formatted (albeit string index)

```
s.index = s.index.format()
s.plot()
```

wonder if we can just register a converter somehow? like #8614 
