Merge pull request just by number

Hi, I'd like to be able to merge PR only by giving their number,
I was wondering if it is in hub philosophy. 

I never wrote ruby before, but I came up with this... nor sure if it is the right way...

Also do you think it could be easy to get a `hub prlist` option that give an output of type :
```
pr number -- mergeable y/n  -- PR title 
pr number -- mergeable y/n  -- PR title 
pr number -- mergeable y/n  -- PR title 
pr number -- mergeable y/n  -- PR title 
...
```

thanks.
\-- 
Matthias