Create a new command to show Pull Request details

[shulard]

We have for the moment a issue:show, I think having a pull-request:show is nice too. It's the same logic than for issue :

• It display general details / description
• A flag --with-comments allow to display comments
• Add tests

[cordoval]

add description txt with a checkbox for add test

[shulard]

Hello,

I've just added a test to validate command execution. This command will be able to dump PullRequest details like :

Pull Request #10 - Write a behat test to launch strategy by weaverryan [open]
=============================================================================

 Org/Repo:         gushphp/gush
 Link:             https://github.com/gushphp/gush/pull/10
 Labels:           actionable, easy pick
 Milestone:        some_good_stuff
 Assignee:         cordoval
 Source => Target: cordoval/gush#head_ref => gushphp/gush#base_ref

Body
----

 Help me conquer the world. Teach them to use Gush.

This PR is ready for a review.

[cordoval]

bump @shulard please rebase so i can review

thanks

[shulard]

I've just rebased on master.