change fs-gh p behaviour, so it includes less steps for PRs:

* `fs gh p` opens current PR (if exists) or new PR page
* `fs gh pulls` opens repo's pull requests
* removed `fs gh p n` in favor of `fs gh p`
add 'p' to fs gh completions
Merge pull request #56 from fs/pimp-my-fs-gh-p

Change `fs gh p` behaviour, so it includes less steps for PRs