Feature request: plot(subplots=levels)
At the moment, the `subplots` parameter in `df.plot()` only supports boolean values: Generate either as many plots as there are columns or generate a single plot with all columns.
 
It would be nice if `subplots` understood level specifications and generate one plot for each unique value in the corresponding level(s) of a multi-index type `columns` index. 

#### Code Sample
Let
```python
import numpy as np
import pandas as pd
df = pd.DataFrame([np.sin(np.linspace(0,y,10))+z for y in [1,2,3] for z in [10,20]]).T
df.columns = pd.MultiIndex.from_tuples([(y,z) for y in ['Y1','Y2','Y3'] for z in ['Z1','Z2']],names=['Y','Z'])
```
```
Y         Y1                    Y2                    Y3           
Z         Z1         Z2         Z1         Z2         Z1         Z2
0  10.000000  20.000000  10.000000  20.000000  10.000000  20.000000
1  10.110883  20.110883  10.220398  20.220398  10.327195  20.327195
2  10.220398  20.220398  10.429956  20.429956  10.618370  20.618370
3  10.327195  20.327195  10.618370  20.618370  10.841471  20.841471
4  10.429956  20.429956  10.776372  20.776372  10.971938  20.971938
5  10.527415  20.527415  10.896192  20.896192  10.995408  20.995408
6  10.618370  20.618370  10.971938  20.971938  10.909297  20.909297
7  10.701698  20.701698  10.999884  20.999884  10.723086  20.723086
8  10.776372  20.776372  10.978656  20.978656  10.457273  20.457273
9  10.841471  20.841471  10.909297  20.909297  10.141120  20.141120
```

The proposed feature would allow
```
[df.xs(val,level='Z',axis=1).plot().set_title(val) for val in df.columns.levels[1]]
```
to be replaced by 
```
df.plot(subplots='Z')
```

More generally, `df.plot(subplots=['Z','Y'])` would draw one plot for each combination of values Z and Y. For the particular dataframe defined above, where 'Z' and 'Y' are the only column levels, this would be equivalent to subplots=True, though it would be a nice bonus feature if the subplots were generated on a grid, with Y varying in the horizontal grid direction, and Z varying the vertical grid direction.

