Automatic column name label with cast
===

When using cast is there a way to set the name of the resulting column? 

Example:

    cast(mtcars, cyl + hp,mean) 
    
       cyl  hp (all)
    1    4  52     2
    2    4  62     2
    3    4  65     1
    4    4  66     1
    5    4  91     2

Instead of `(all)`, I'd like to be able to set the name within the call to cast. Is this possible? 

I know, I know, it's as if typing `colnames(x)[3] <- "Foo"` is too difficult, but it's time consuming if you have to do it frequently!

