ENH: Incorporate inplace and subset parameters into pandas.DataFrame.apply
I would love to see pandas.DataFrame.apply incorporate the inplace and subset parameters.  Normal operations when inplace is not in use would still apply.  Subset should only be used if inplace=True is provided.

My lame examples of how the function would look if implemented.  

```
import numpy as np
import pandas as pd

# Set the seed for a reproducible sample
np.random.seed(0)  

df = pd.DataFrame(np.random.randn(10, 5), columns=list('ABCDE'))

df.apply(lambda x: x + 2*x, inplace=True) # would apply x + 2*x to each column
df.apply(lambda x: x + 2*x, inplace=True, subset=['A','B']) # would apply x + 2*x to each column, 'A' and 'B'
```