API: Accept negative value for layout keyword in plot

Once https://github.com/pydata/pandas/pull/7736 is merged, we can add this option

```
df.plot(subplots=True, layout=(4, -1)
```

This will behave similar to numpy's `reshape` method, except that we won't require the number of subplots generated by layout to exactly match the number of subplots in `df`. We'll just fill in the smallest integer that' sable to hold the subplots.
