Ability to assign pull request to a github user

My usual workflow when creating a pull request is to assign it to a collaborator. Right now, using `git hub request create` doesn't save me much time because I still have to open up github.com to assign the PR. It would be great if I could pass an `-a ` parameter to `git hub request create` to automatically assign the PR when it's created.

Start from the implementation and argument parsing for `git hub request create`; the issue does not name a file or test. Add support for `-a ` so the created GitHub pull request is assigned to that user, then verify the command creates an assigned pull request.

Written by the indexing model from the issue text.