ENH: Ability to use "normalize" function (df['col'].value_counts(normalize=True) when you do df.groupby(cols).size()
`groupby.size()` should have the ability to "normalize" the results and return them as a percentage.   
  
  
```
df['col'].value_counts(normalize=True) 
  
A = 0.25
B = 0.25  
C = 0.25  
D = 0.25
```  
  
To accomplish this result with a `groupby.size()` you have to do the following - 

```
df2 = df.groupby(['subset_product', 'subset_close']).size().reset_index(name='prod_count')
a = df2.groupby('subset_product')['prod_count'].transform('sum')
df2['prod_count'] = df2['prod_count'].div(a)
```  
  
Is there a way to add a feature to include this as a parameter in `.size()`