Add the functionality to memorize "temporal variable" to use it after chain evaluation

Hi;
Thank you for your great package.
I'm really fascinated by your approach(and F#?).

I try to introduce the functionality to "memorize" temporal variable to use after the chain evaluation.
I think that it is very usefule when you chain data manipulation and visualization 
because processed data will be needed sometimes after chain evaluation.

For example, you can write like the following using "%->%" operator I made.
```r
x <- 1:10
x %>% sum %->% y %>% multiply_by(10)
y
(output)
[1] 55
```
And as second example,
```r
iris %>% group_by(Species) %>% summarise(mn=mean(Sepal.Length)) %->% res %>% qplot(data=.,x=Species,y=mn,geom="bar",stat="identity")
res
(output)
Source: local data frame [3 x 2]

     Species    mn
1     setosa 5.006
2 versicolor 5.936
3  virginica 6.588
```

Let me know what you think.