Add org flag to pull-requests command

I've added an `-O` flag to pull-requests for specifying an organization. I added this so I can get a list of pull requests I have open on my organization's repos. This is pretty rough, but I wanted to open up a conversation around this and see what your thoughts were about this functionality and the proper way to implement it.
 
##### Caveats

* This is only working under the `--all` flag

##### Usage

* To get all pull-requests for your organization run `gh pr -a -O <org name>`
* To get all pull-requests for your organization that _you've_ opened run `gh pr -a -m -O <org name>`